http://www.ownedcore.com/forums/worl...reenshots.html
The secret watermark which is being intentionally embedded inside WoW generated screenshots below top quality, DOES NOT CONTAIN the account password, the IP address of the user or any personal information like name/surname etc. It does contain the account ID, a timestamp and the IP address of the current realm. It can be used by hackers to link alt. characters to accounts and target specific spam or scam attacks, and it can be used by Blizzard to track down private WoW servers.
No confirmation if the same is true of StarCraft 2 or Diablo III screenshots. It's a good argument against posting screenshots of you using 'sploits or a private server, if nothing else. At least not without using a 3rd party capture tool like FRAPS. Apparently setting your screenshot quality to 10
Code:
/console SET screenshotQuality "10"
will also rid you of the watermark.