The Catholic Church has a great deal to say about prayer. Most relevant to the question, I think, are Expressions of Prayer found in the Catechism of the Catholic Church (CCC).
The Lord leads all persons by paths and in ways pleasing to him, and
each believer responds according to his heart’s resolve and the
personal expressions of his prayer. However, Christian Tradition has
retained three major expressions of prayer: vocal, meditative, and
contemplative. They have one basic trait in common: composure of
heart. This vigilance in keeping the Word and dwelling in the presence
of God makes these three expressions intense times in the life of
prayer. (CCC 2699)
Firstly, we're talking about three expressions of prayer. This is notable. Prayer is an expression of the uplifting of ourselves to God. That said, the three expressions:
Vocal
We pray with words, mentally or vocally, giving our prayer "flesh."
Meditatieve
According to the Catechism, "There are as many and varied methods of meditation as there are spiritual masters. ... Meditation engages thought, imagination, emotion, and desire."
Contemplative
The Catechism says, "Meditation engages thought, imagination, emotion, and desire. ... Contemplative prayer is hearing the Word of God. ... Contemplative prayer is silence, the “symbol of the world to come” or “silent love.”"
Now, while I'd definitely suggest reading the whole article entitled Expressions of Prayer to get a better sense for what we really mean by these expressions, your question lingers primarily in meditative prayer, wherein our imagination is active. But, exploring each a little:
In vocal prayer, we're speaking to God in response to His Word. In a purely vocal prayer, imagination isn't really required. Whether we're capable of a strictly vocal prayer is debatable -- an imaginations is hard to turn off. But, in the most basic sense, we can pray vocally as though God is an unseen and mysterious person (or three) standing around the corner, listening intently. We don't have to imagine anything, really. We're just aware that our words, mental or vocal, express what we wish to say (presumably to God).
In meditative prayer, we take steps to engage more of our mental faculties to express ourselves, and perhaps more importantly to hear God. Thus, we imagine God, His Word, scenes from the Bible from various perspectives, we ponder the lives of the saints, imagine ourselves speaking to God, etc.. I understand this prayer largely as an explorative prayer: with our imagination, emotion, and thought, we seek or feel around for God and His Word. It's important in this sort of prayer, as with any, to avoid clinging to any particular image of God or revelation, checking/validating all things against Biblical wisdom and the general teachings of the Church.
In contemplative prayer, we sort of silently groan for God, in response to His call for intimacy with us. We unite with Him in a way that no other form of prayer can bring, because this prayer is 100% grace. Our role active, but it's an active "yes" to God's will to bring us into contemplative prayer. As such, where our minds and hearts (and bodies) go in contemplative prayer is beyond recommendation -- God takes them where entirely He wills.
In brief, your question centers primarily on meditative prayer. There are a lot of recommended methods and approaches for meditation. All of them, as far as I'm aware, are explorative efforts, wherein the valid fruits of our labor will square with our conscience, scripture (Jesus Himself), and the Church in general.
