What is the character of an imagined scene?

I believe that the best we can say about people's visual imagination is that there's a continuum. Some people have no visual imagination at all, and then there are people with eidetic memory so advanced I hazard to say they need to "imagine" anything at all, pun intended.

I've been thinking about this recently because my grandmother is dying and it bothers me that I can't imagine what she looks like. Or anyone, for that matter. All I end up imagining is a fragment of her hair, an eye, or some other piece of her face. It's disconcerting.