The word visualize refers to making an image visual, seeing it as if a picture you are looking at. This is the same as "picture it in your mind's eye". Simply knowing what it looks like is not actually "picturing it", not seeing it as an image in the mind.

I am able to do this quite easily. I just pretend that I am looking at a carrot and try to see it and an image forms in the mind. Doing this is a way of guiding and controling one's imagination. It can be a useful spiritual exercise.