We have heard from Leanpub authors that sometimes in their browser, it does not work when they try to upload an image for their profile.
(Please note that currently, if you are not an author, there is no option to add an image or avatar, since only authors have profile pages.)
If this is happening to you, here is a method that may work:
Under "UPLOAD YOUR AVATAR", click "Choose File", and select the file on your computer.
Then, click "Choose File" again, and select the file on your computer again.
Scroll to the bottom and click "Save Settings". Then, your image should appear.