In my experience mental images are quite foggy most of the time. I can see the overall shape and maybe two or three details, but that’s it.
Some time ago I figured out a method to sharpen these pictures that works at least for me. It is a modified version of Win Wengers image streaming. It indeed worked so well that after just a few days my images turned into kind of pseudohallucinations. They were as vivid as real objects, hanging in the air in front of me even with open eyes. For example I saw the head of a cheetah, including the exact pattern of the fur. It was so creepy that I got scared and stopped.
Sure, enhanced visualization sounds cool, and maybe it helps to improve memory. But since then I think that it can also be a curse.