Neat. Your conclusions don't surprise me much, but I hadn't heard of the evaluating-your-mental-imagery exercise before. It's obvious to me now that my mental images are very foggy; they're mostly concept with a small amount of visual detail attached. However, I'm sure this a faculty that can be improved. Before I started my music training, songs stuck in my head were tuneless, basically the lyrics in rhythm but in a kind of monotone. After 7 years of being in bands and choirs, I can now hear tunes in key in my head (imagining melodies involves feeling them in my throat a little, as if I were singing them) and I can even "hear" chords by superposing several notes. (Composers such as Mozart had incredible capacities to do this, which I will probably never be able to mimic...my imagined melodies still lack "texture" i.e. the difference in sound between a flute, trumpet, voice, etc). Presumably I or anyone else could do the same with mental images by doing some kind of art training.

As an aside, in a novel I wrote, there is a community of children that use echolocation to 'see' objects underwater in flooded houses.