Avatar upload during register. D6
I haven't used it before but the Register with Picture module sounds like it would do the trick:


  This module exposes the 'upload picture' element to the registration form. It also gives you the ability to set a picture as required in the module settings.


There are stable versions for both Drupal 6 and Drupal 7. It also has 1600+ installs, is actively maintained and has very few issues in the queue, so I'm assuming it should work great.