When uploading, indicate that the file to be uploaded has been 'saved' to the site, usually done by inserting the filename on screen, just above the 'upload file' button.
Indicate that avatars will be cropped to round shapes (or by adding an optional cropping tool)
Indicate that only *.jpg files are allowed (was unsuccessful at uploading PNG images)
I also found a bug, when trying to upload a second avatar, it is not 'saved', the first avatar remains the default avatar.
(Edit: But then I logged out and back in, and it updated correctly)