Weird... I actually tried with a couple of different browsers (FF, Chrome and Safari) to check that it wasn't me logged in or something like that. Could it be ip-based, as I did all my tests with the same computer? Hmm...

Anyway, thanks for expanding on it; I've shared a few images through imgur and never had this issue (maybe it was before this change, or maybe the people receiving them never complained), so I was a bit puzzled by your comment.