I'd have thought that techncally Les B's suggestion would be quite possible. Discussion forums often have an option to have a "avatar" icon selected by a member appear in that way, and I can't see why using a small photo-image need be much different.

But I think I'd be against it - I think it'd be likely to tip over the balance a bit far towards putting the emphasis on the poster rather than what they posted. A compromise would be to have some indication next to the name of the person posting that would indicate if they'd got a picture on file.