I can't consciously form mental images, but I have no problem daydreaming images which seem to come to my mind randomly, and I do sometimes have vivid dreams.

I'm sure that forming mental images can be improved with practice. For instance, people who play a lot of chess or Go can visualize the board in their head and the relationship between the pieces, to the point where they can play a game entirely in their head.

When I try to visualize a mental image, the pieces of the image just don't stay there. For instance, say I try to visualize a house with flowers and a porch and trees and children playing in the yard, and so forth. (I just tried this now to see what happens in my mind.) When I put the porch down, and then try to put some trees in and visualize all the details, the porch "disappears" and I have to remember how I built it. I just don't understand how anyone has a good enough memory to construct a persistent mental image. To me, it's like holding ten phone numbers in your mind.