Yes regarding WoW ,Quote from overneathe
Blizzard has this on all their games for some time now (2008?). They have embedded code in each screenshot that identifies which game and server that player is playing on, which client they're using (32/64) and a long string of numbers identifying the player, but one that only Blizzard can read that has no personal or game info attached.
It's mainly used to hunt down exploiters/leakers, since when they post a screenshot it's easy to find out exactly which player it is. No outside source can use that info though.
It's not very popular and talked about since, well, you know, if everyone knew about it - it wouldn't really work. But it sadly got pretty well known about a year ago.
But this is regarding Diablo 3 haven't seen anyone post anything about watermarks in diablo 3