Some people, especially when they feel very down or hopeless, mentally look during prayer at an image of Our Lord on the cross or some other images.
Generally speaking, the use of imagination during prayer is discouraged because imagination, being a parasite of Adam’s fall, is the portal through demons can (and will) attack.
Sometimes, especially when we are in the two situations described above or at the beginning of the spiritual life or during a sorrow, we can use this technique, but we must take care to not be accustomed with it. The pure prayer is the prayer without images. God is purely spiritual and our prayer should be as such, because the prayer is the union of mind with God. It is not about which is the “proper” image to use – we should close our mind in the words of the prayer without any image because otherwise our mind instead being united with God and have the desired effects, will take the shape of the thought(s) and in fact the mind talks with them.
If we are outside of God’s grace, the contemplation of creation can help. If God’s grace acts upon us, especially during prayer, we should not lose the higher thing for the lower one.
It is a very common parasite called “wandering of the mind”. There is an extensive therapy about this but it takes time. In any case, it is worth the results.
Based on Saint Mark the Ascetic, Saint John of the Ladder, Elder Joseph of Vatopaidi