PHP/Image Questions, Selectable Avatars..?

I found PunBB last night, and fell immediately in love. It's so small, quick, efficient, flexible.. exactly what I've been searching for for the longest. Bar none, the best I've found so far.

However, the whole time I've been searching, one of the main features I wanted was avatars.. in the most basic sense. I would like for my users to be able to choose from a list of avatars I've already made, as opposed to remote linking or even uploading 'em.

I know that this has got to be easily implementable (in theory; changing the "Upload avatar" to "Select avatar" and displaying a list of the available avatars, each hotlinked to select their URL in a hidden <input>.. or something), and having it available as a mod would make PunBB a dream come true. Thing is, I know next to nothing about implementing the concept in PHP. If anyone out there with a bit of knowledge could help me out with some pointers in the right direction script-wise, or some example, I'd be extremely grateful.

Thanks in advance!