FYI, the phenomenon you describe is an afterimage and happens to everybody (http://en.wikipedia.org/wiki/Afterimage). You are correct that the visual cortex is involved in both phenomena but they are quite different. An after-image is still an "upstream" image on your retina, whereas a mental image is not. Whether the former can be used to increase facility with the latter I do not know. I haven't seen scientific evidence that imagery can be trained at all.
Despite all that it doesn't sound like a terrible place to start.
Ah, that's what it's called, thanks! I read about that a while back but forgot whether it was a phenomenon of the retina or of the brain. What I am wondering is if you can use the after image as almost a skeleton for more advanced visualizations. For example, imagine the retinal after image laying down the basic shape, and then you can consciously add detail on top of that after image. Maybe using the after image as a kind of training wheels..if it's even possible..
Despite all that it doesn't sound like a terrible place to start.