I have access through the Drupal Who's Who as a Membership Admin. I get the same page you do when you click "My Account" and upload the exact same way you would. For some I need to delete the original avatar first. For others I have to reopen the avatar in Paint and resave over the original...
There is no real reason there should be a problem, I don't even do anything different most of the time. Like in Badria's case it was a simple upload. Occasionally, I have to reopen it in paint, resave, then upload but that's the extent of it.
Done Badria :)