Do you mean as part of the theme? In other words to give your members more of a feeling like the site is their site, like some mutli-level companies do or do you mean more in the way that dating web sites do?

The overall theme idea I have though of I see it working this way.

1. you have to limit them to one theme.
2. You have to find a way to load a picture bases on the login name.
If login name is this , Load login+mypic.jpg Right,

This is as far as my thoughts have gone on this and I am very new so what I have suggested may not be feasable