Just to clarify, are you trying to use gravatar for your profile, or you'd like to have a project logo for your repository? Title says change profile picture whereas in issue description you're talking about project logo.
The trick is, you don't actually have to DO anything except click the 'save' button on your account page. Clicking the link to gravatar does nothing. It does not 'load' automatically when you log in, you have to click that save button. (confusing as heck, btw.)