If someone saw a nude image ad once on the internet, would that be considered sin in the Catholic church? What about a nude painting at an art gallery? Is it a mortal sin to see these? Or should one talk to their parish priest about the matter?
Is seeing nudity a mortal sin in Roman Catholicism?
The short answer is: Yes and No. It depends on the situation.
In the Catholic Church, sins come in two basic types: mortal sins that imperil your soul and venial sins, which are less serious breaches of God’s law. The Church believes that if you commit a mortal sin, you forfeit heaven and opt for hell by your own free will and actions.
Three conditions are necessary for mortal sin to exist:
Grave Matter: The act itself is intrinsically evil and immoral. For example, murder, rape, incest, perjury, adultery, and so on are grave matter.
Full Knowledge: The person must know that what they’re doing or planning to do is evil and immoral.
Deliberate Consent: The person must freely choose to commit the act or plan to do it. Someone forced against her will doesn’t commit a mortal sin.
Venial sins are any sins that meet one or two of the conditions needed for a mortal sin but do not fulfill all three at the same time, or they’re minor violations of the moral law. - Mortal and Venial Sins in the Catholic Church
Seeing nudity in itself is not sinful. But under certain circumstance it may be sinful and even mortally sinful.
It is not sinful for example to see your own naked body when you are bathing or taking a shower.
Spouses often see one another naked and no sin is ever committed. It is a natural part of Christian marriage
It is not sinful for a doctor to se a patient naked in his medical office or operating room while perform some medical procedure. It could be sinful if a doctor abuses his medical privileges for some sexual pleasure.
It is not a sin to see to an artistic piece of art of some personage in the nude. However it becomes sinful if one deliberately does so in order to arose themselves sexually.
Watching pornography is always seriously sinful matter and is thus a mortal sin.
If someone saw a nude image ad once on the internet and did not deliberately remain looking at it, no mortal sin would have been committed. However, if one deliberately remained looking at it, one may have committed at least a venial sin. If one remained looking at it for some sexual gratification then a mortal sin would have been committed. It is always best to see a priest in the confessional to make sure one is in the state of grace at all moments.
Let us finish with the definition of sin and the differences between mortal sin and venial sin. The Catechism of the Catholic Church should be a good source for this and it is given here in it's entirety.
II. The Definition of Sin
1849 Sin is an offense against reason, truth, and right conscience; it is failure in genuine love for God and neighbor caused by a perverse attachment to certain goods. It wounds the nature of man and injures human solidarity. It has been defined as "an utterance, a deed, or a desire contrary to the eternal law."
1850 Sin is an offense against God: "Against you, you alone, have I sinned, and done that which is evil in your sight." Sin sets itself against God's love for us and turns our hearts away from it. Like the first sin, it is disobedience, a revolt against God through the will to become "like gods," knowing and determining good and evil. Sin is thus "love of oneself even to contempt of God." In this proud self- exaltation, sin is diametrically opposed to the obedience of Jesus, which achieves our salvation.
1851 It is precisely in the Passion, when the mercy of Christ is about to vanquish it, that sin most clearly manifests its violence and its many forms: unbelief, murderous hatred, shunning and mockery by the leaders and the people, Pilate's cowardice and the cruelty of the soldiers, Judas' betrayal - so bitter to Jesus, Peter's denial and the disciples' flight. However, at the very hour of darkness, the hour of the prince of this world,126 the sacrifice of Christ secretly becomes the source from which the forgiveness of our sins will pour forth inexhaustibly.
III. The Different Kinds of Sin
1852 There are a great many kinds of sins. Scripture provides several lists of them. The Letter to the Galatians contrasts the works of the flesh with the fruit of the Spirit: "Now the works of the flesh are plain: fornication, impurity, licentiousness, idolatry, sorcery, enmity, strife, jealousy, anger, selfishness, dissension, factions, envy, drunkenness, carousing, and the like. I warn you, as I warned you before, that those who do such things shall not inherit the Kingdom of God."
1853 Sins can be distinguished according to their objects, as can every human act; or according to the virtues they oppose, by excess or defect; or according to the commandments they violate. They can also be classed according to whether they concern God, neighbor, or oneself; they can be divided into spiritual and carnal sins, or again as sins in thought, word, deed, or omission. The root of sin is in the heart of man, in his free will, according to the teaching of the Lord: "For out of the heart come evil thoughts, murder, adultery, fornication, theft, false witness, slander. These are what defile a man." But in the heart also resides charity, the source of the good and pure works, which sin wounds.
IV. The Gravity of Sin: Mortal and Venial Sin
1854 Sins are rightly evaluated according to their gravity. The distinction between mortal and venial sin, already evident in Scripture, became part of the tradition of the Church. It is corroborated by human experience.
1855 Mortal sin destroys charity in the heart of man by a grave violation of God's law; it turns man away from God, who is his ultimate end and his beatitude, by preferring an inferior good to him.
Venial sin allows charity to subsist, even though it offends and wounds it.
1856 Mortal sin, by attacking the vital principle within us - that is, charity - necessitates a new initiative of God's mercy and a conversion of heart which is normally accomplished within the setting of the sacrament of reconciliation:
When the will sets itself upon something that is of its nature incompatible with the charity that orients man toward his ultimate end, then the sin is mortal by its very object . . . whether it contradicts the love of God, such as blasphemy or perjury, or the love of neighbor, such as homicide or adultery. . . . But when the sinner's will is set upon something that of its nature involves a disorder, but is not opposed to the love of God and neighbor, such as thoughtless chatter or immoderate laughter and the like, such sins are venial.
1857 For a sin to be mortal, three conditions must together be met: "Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent."
1858 Grave matter is specified by the Ten Commandments, corresponding to the answer of Jesus to the rich young man: "Do not kill, Do not commit adultery, Do not steal, Do not bear false witness, Do not defraud, Honor your father and your mother." The gravity of sins is more or less great: murder is graver than theft. One must also take into account who is wronged: violence against parents is in itself graver than violence against a stranger.
1859 Mortal sin requires full knowledge and complete consent. It presupposes knowledge of the sinful character of the act, of its opposition to God's law. It also implies a consent sufficiently deliberate to be a personal choice. Feigned ignorance and hardness of heart do not diminish, but rather increase, the voluntary character of a sin.
1860 Unintentional ignorance can diminish or even remove the imputability of a grave offense. But no one is deemed to be ignorant of the principles of the moral law, which are written in the conscience of every man. The promptings of feelings and passions can also diminish the voluntary and free character of the offense, as can external pressures or pathological disorders. Sin committed through malice, by deliberate choice of evil, is the gravest.
1861 Mortal sin is a radical possibility of human freedom, as is love itself. It results in the loss of charity and the privation of sanctifying grace, that is, of the state of grace. If it is not redeemed by repentance and God's forgiveness, it causes exclusion from Christ's kingdom and the eternal death of hell, for our freedom has the power to make choices for ever, with no turning back. However, although we can judge that an act is in itself a grave offense, we must entrust judgment of persons to the justice and mercy of God.
1862 One commits venial sin when, in a less serious matter, he does not observe the standard prescribed by the moral law, or when he disobeys the moral law in a grave matter, but without full knowledge or without complete consent.
1863 Venial sin weakens charity; it manifests a disordered affection for created goods; it impedes the soul's progress in the exercise of the virtues and the practice of the moral good; it merits temporal punishment. Deliberate and unrepented venial sin disposes us little by little to commit mortal sin. However venial sin does not break the covenant with God. With God's grace it is humanly reparable. "Venial sin does not deprive the sinner of sanctifying grace, friendship with God, charity, and consequently eternal happiness."
While he is in the flesh, man cannot help but have at least some light sins. But do not despise these sins which we call "light": if you take them for light when you weigh them, tremble when you count them. A number of light objects makes a great mass; a number of drops fills a river; a number of grains makes a heap. What then is our hope? Above all, confession.
1864 "Therefore I tell you, every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven." There are no limits to the mercy of God, but anyone who deliberately refuses to accept his mercy by repenting, rejects the forgiveness of his sins and the salvation offered by the Holy Spirit. Such hardness of heart can lead to final impenitence and eternal loss. - The Catechism of the Catholic Church
Every Sin originates from the fall of man in some way.
"So when the woman saw that the tree was good for food and that it was a delight to the eyes, an that the tree was to be desired to make one wise, she took of its fruit and ate."
Saw that the tree was good for food - Lust of the flesh the tree was "a delight to the eyes" - Lust of the Eyes desired to make one wise - Pride fo life
Repeated again in John 2-16, the very source of our disobedience stems from these words, every sin comes from these three.
To answer your question, When you saw the naked person, did you experience "LUST and Desire" and then did you consciously feed on that Lust and desire?
A person can see beauty without Lust and without desire. It can also feel the attraction and fight against it, using the will to be obedient and overcome the desires of the flesh.
In Catholicism, we are taught to fight our desires, the methods that we incorporate, especially during the Lenten season but not limited to that time are Fasting, almsgiving and prayer. These three things run counter to the three elements of the Fall. When you fast you fight and reject the lust of the flesh, When we give alms, it is the opposite of seeing with our eyes and taking, but seeing others in need and giving. When we Pray, it is a death blow to Pride, as when we pray we are saying to our Heavenly Father, you are God and I am not.