I tried visualizing some images now -- and it seems harder than I expected, seems harder with eyes closed (makes eveyrthing feel too dark and low contrast). But I think my dreams are more vivid than that.
Also I relized that when actively trying to imagine things, the "resolution" or sharpness/focus of the mental picture is very narrow -- I can only imagine the "details" of texture/surface of things when focussing attention on a very small and narrow field of "vision".