I will try colors and natural scenes and (like Elithrion suggested) familiar full-featured scenes.

Other senses don't transfer, that I know. I have good aural and motor "visualization".

Edit: I didn't get colors to work at all. Real scenes, especially familiar ones, and scenes that were emotionally strong, yielded some results. I got to something like 3.75 on the scale (close to vague and dim but on the way to moderately clear and vivid) for lots of locations that I visit/see regularly. When I noticed that I was better-than-4 on lots of things, I tried an apple, and after a couple minutes got to maybe 3.5ish for a second or two. I feel like I can do one assisted pull-up now, which is sufficient to start training. :D