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Technically we're actually creating a new resource when you "update" your avatar so POST makes sense for us in that context. The alternate response is, we're not trying to adhere too strictly to REST :)
Due to the nature of REST and the fact that you're 'updating' your avatar, surely the endpoint should be a PUT rather than POST?
http://developers.app.net/docs/resources/user/profile/#update-a-users-avatar-image