My current project is to build a face authentication system. The constraint I have is: during enrollment, the user gives single image for training. However, I can add and us
I would recommend that you give SOM(self-organizing maps) a close look. I think it contains the solutions to all the problems and constraints you have mentioned.
You can employ it for the single image per person problem. Also, using the multiple SOM-face strategy, you can adapt it for cases when additional images are available for training. Whats pretty neat about the whole concept is that when a new face is encountered, only the new one rather than the whole original database is needed to be re-learned.
A few links which you might find helpful along the way:
http://en.wikipedia.org/wiki/Self-organizing_map (wiki)
http://cs.nju.edu.cn/zhouzh/zhouzh.files/publication/tnn05.pdf (An interesting research paper which demonstrates the above mentioned technique)
Good Luck
To make your classifier robust you need to use condition independent features. For example, you cannot use face color since it depends on lighting conditions and state of a person itself. However, you can use distance between eyes since it is independent of any changes.
I would suggest building some model of such independent features and retrain classifier each time person starts authentication session. Best model I can think of is Active Appearance Model (one of implementations).