I just finish browsing 10 pages on the search engine for a problem with the Avatar too, and i am sure i read in the past about someone having exactly this problem.
Basically, after a fresh reinstall, i now get new members uploading their picture as My picture!!! every new member joining in, change the administrator's picture.
I then promptly went to put back the admin's avatar, but was not there anymore, i choose to use one of the default's avatar but as soon as i select it, another picture of another user (with admin's privileges) has loaded up, i removed it, but this cancelled the avatar of this other admin!!! Total madness, i am going to try to disinstal and reinstall the avatar module again to see if there are any improvements....
Panic 
Edit:
No, reinstalling the Avatar's module and recompiling the language didn't fix the problem.
Basically, as soon as you finish cropping a picture to use as the avatar, a new picture of someone else appear on the next window, asking if you want to upload that.
Even if i just ignore this new random picture coming up, it will just decide from itself to be the default avatar....