Saint Theophan says,
Hold no intermediate image between the mind and the Lord when practicing the Jesus Prayer.Elder Porphyrios says,
Pray without forming images in your mind, Don't try and imagine Christ.... With an image, the foccus of prayer is eaily lost, because one image can eaisly be displaced by another. And the evil one may intrude images and we lose the grace.... Don't think anything but the words "Lord Jesus Christ, have mercy on me."Saint Theophan even goes further highlighting that the words of the prayer are useful but only a help. Even they are not the essence of prayer. He says,
The words pronounced are merely a help and are not essential. The principlal thing is to stand before the Lord with the mind in the heart.He is not telling us not to use the words, but is helping us see the essential aspect of prayer. He continues,
The essential part is to dwell in God, and the walking before God means that you live with conviction ever before your consciousness that God is in you, as He is everything: you live in the firm assurance that He sees all that is within you, knowing you better than you know yourself.This is a powerful thought. God knows all. There is no way we can hide anything from Him. But his can raise images of God in our mind. Saint Theophan warns us to not let this bring up any images of God.
This awareness of the eye of God looking at your inner being must not be accompanied by any visual concept, but must be confined to a simple conviction or feeling....Yet, he also tells us that the words, even though they are not the essence of prayer, they are still important.
The words "Lord Jesus Christ, Son of God, have mercy on me" are only the instrument and not the essence of the work; but they are an instrument which is very stong and effective, ...
Do not forget that this practice is simple, and must not have anything fanciful about it."If we are not careful in our prayer, we can let images limit our effectiveness of prayer. In prayer we are relating at a spiritual level where there are no forms like in this world.
The advice is: Don't let images hold you prisoner to the things of this world. Allow your soul to soar. If images arise, gently discard them.
Source: Christ the Eternal Tao pp 349-350