Great info, I didn't know this either.
They also mention:
"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 malicious 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."