What a great question, I had never put an image of God in my mind while praying. Francis says that while he is praying he imagines Paul's interpretation of what God looks like; Paul tells us that God in all His glory is a magnificent being, clothed in beautiful jewels of every color, surrounded by thunder, fire and angels. Now this image (for me) is not very approachable. If I were to pray with this image in my mind, I would be tongue tied.
Tyson had asked me this question quite a while ago and my little Sonny Man answered it so very beautifully;
Tyson: What does God look like Mommy?
Sonny: He looks like the stars in the sky, he looks like you & me & mommy & daddy & the trees & the sun & the moon.
Tyson: Where does he live?
Sonny: In your heart.
You are a marvelous mystery!