A group of users on the OwnedCore forums discovered that screenshots taken in Blizzard's popular MMORPG World of Warcraft include watermarks that identify information like the account ID of the player who took the screenshot, a timestamp of when the screenshot was taken, and the IP address of the server the player is connected to.
Sendatsu, the OwnedCore member who compiled all the research done by the OwnedCore community, points out that this information can be used by hackers to identify the accounts of alternate characters and target users for attacks. Blizzard can also use this information to discover and track down private World of Warcraft servers that aren't being run and administrated by Blizzard itself.
The thread on OwnedCore includes instructions for revealing the watermarks in screenshots. The process recommends visiting the Crystalsong Forest, as the white trees provide an effective backdrop for finding the watermarks. After taking a screenshot, sharpening the image a few times using an image editing application will reveal the watermark, which is unique to each character.
The watermarks have been inserted in screenshots sometime between 2008 (Patch 3+) and 2010 (Patch 4+), which is after Activision acquired Blizzard. However, the watermarks are not included in high-quality screenshots taken from the game, so to prevent watermarks from being inserted into screenshots, you can simply run this command to set the screenshot quality to maximum:
/console SET screenshotQuality "10"
Source: OwnedCore Forums