In the book of Deuteronomy Chapter 4:15
Therefore take good heed to yourselves. Since you saw no form on the day that the Lord spoke to you at Horeb or of the midst of the fire, beware lest you act corruptly by making a graven image for yourselves, in the form of any figure, the likeness of male of female.
God, at this time, had not revealed to us his image, and Israel having shown a desire to make images and worship them as God, God commanded them, informing them why, the reasons you cannot is that I have not revealed to you my image.
Short of this explanation, the cherubim, the serpent, the artistry in the temple would be a cause of confusion to the Jews. However, like the Church Christ founded, there is no confusion of what God meant when he spoke to Israel.
There came a time later however, that the confusion entered into people of God and the Serpent had to be destroyed as some were making offering to it.
He removed the high places and broke down the sacred pillars and cut down the Asherah. He also broke in pieces the bronze serpent that Moses had made, for until those days the sons of Israel burned incense to it; and it was called Nehushtan. 2 Kings 18:4
The Nehushtan was a relic and they were worshiping it as God in a manor as the eqyptians did, in that god was dwelling in the Serpent.
When God became man something changed concerning images. For Moses said we should not make images God having not revealed himself, the fulfillment of this instruction was the Incarnation. God now having been revealed to us we are free and encouraged to express the image of his son and the once for all time sacrifice he made for us.
It the book of Acts, Paul sees a monument to an unknown God, yet we have known him, we preach Christ Crucified and we preach Christ on the Cross, for a Cross empty of Christ is empty of power and just 2 sticks.
The Greatest example of Gods self Gift (Love) is Christ revealed to us on the Cross, when a Catholic kneels before it, or any of the Saints it honors we do not worship the wood or the stone or the Gold or the silver but the Love that that image represents, Christ himself.
We are allowed to have images, because Christ the very image of God has been made known to us, he did not speak through a pillar of flame but as the visible person of Christ.
why is it allright to have images when previously we could not, I would simply say that God was speaking to those who he did not reveal his image to. Now, having revealed himself to us the commandment of making images no longer pertains to us, as the reason that was given no longer exists.
He was still speaking, when lo, a bright cloud overshadowed them, and avoice from the cloud said, "This is my beloved Son, with whom I am well pleased" (the image of God Revealed) When the disciples heard this, they fell on their faces, and were filled with awe. But Jesus came and touched them saying, "Rise, and have no Fear"
We Catholics have no fear when we kneel before a Cross with Chirst on it, for we know that he has revealed the image of himself to us.
So, if you want to understand why Images are ok, simply read the scriptures and see why they were forbidden in the first place it is right there for you to read. Reading was a luxury that many Christians didnt, Before the advent of the printing press people relied on images and sculptures and stained glass and everything you can think of to be evangelize and to be evangelized.