Drag and drop is intuitive from nothing. This is a change of an expected behavior to something very different from before. No first-time pop-up message for returning users, no backup "edit picture" button, nothing. That is poor design. Sure, now that I know about, it works great, but it should be made clear before the user gets to this point.
I agree fully about the new way to do this being really unintuitive. I bought an MBA 13 two weeks ago, which came with Lion installed. Setting it up was relatively straightforward, even though the old Macs in our house were running Leopard, so I had some learning to go through.
The MBA died after 10 days, before I had a chance to install Mountain Lion. I replaced it with a MBP 13 today. The Apple support information, OS X Mountain Lion: Add a user picture, still describes the procedure for Lion, makes no mention of this, and is useless. I just wasted an hour trying to figure this out, before stumbling on the answer in this thread (thank you, Fernando.S, for the answer).
At the moment, I am disgusted with Mountain Lion, which has thrown up one obstacle after another.
Just so you know, this is not the new way of doing it. It's always worked this way. And the method used, dragging and dropping, has been an essential part of the Mac OS since 1984.
It's probably not intuitive to some ex-windows users, because in 1984, they were still using menu commands in DOS. IMHO