In any app that has user avatars, users should be able to change those avatars. Anything to make that easier is desirable. Many apps start with a user's Twitter avatar, Facebook avatar, or Gravatar. That's a smart move. Avatars give users a sense of ownership over a virtual space so any way to get them to have their desired avatar is good for engagement.
Let's create a page where a user can update their avatar with as little friction as possible: they just drop an image anywhere on the page and it's done.