I can't think of anything that's one word that's distinctive over any other kind of picture. It needs to be something that's specific to an online profile or account.
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I think it could be described as one of the following:
I would not use avatar as a synonym as that would reference what is usually not a picture of yourself, but an icon of some sorts. My avatar is a picture of rye bread, the stack exchange sites have a random icon generator for avatars. Some sites do a collection of robots or weird puppets. Again my opinion is that avatar usage is different from profile picture.