Module to change filename of uploaded items

Hi

after searching the forums for something related I decided to post this question:

how would I best go about writing a module that changes the filename of uploaded items? The default is to use the uploaded filename, but that annoys me, since my digital camera recycles the counter every so often, so I end up with different photos with the same filename on disk. I know that this doesn't really matter since the photos are all in subdirectories, but ... I also want to learn how to write modules for g2.

Problem is, I don't even know where to start. I am sifting through the source right now, but don't want to just "hack" the code that moves the uploaded items into place, I want it to be g2 conform.

Any pointers?