This is because if you are using mostly the part of the brain that contains words than the other part that contains images. When you are able to sit down quiet, then the brain automatically can start it’s idle process and bring more information about what was previously accessed, which was words.
So in order to think of images you can actually start thinking of images, such as reading story books (see thousands of free public domain books from the gutenberg website), later it would be audio books. Perhaps start a collection of images as well, such as shaders, game screenshots, 3D models, etc…
I remember that two years ago all I saw was a black canvas and nothing more, I was very frustrated at some point where I looked into the subject and read a book “pragmatic learning and thinking” which explained these concepts in detail. Now I can think of images normally again (now for no reason I thought the first level of Ace Combat 3 in PS1 … perhaps it’s about time to play again this one).