From reading the thread, the artifacts do not appear when JPEG quality is set to 10 (i.e. maximum) or if a non-lossy algorithm is used (like TIFF or PNG). If this was meant to be a watermark, the programmer who wrote the algorithm should be fired.
These are most likely JPEG compression artefacts.
They did this on purpose, in order to avoid having their watermark identified when viewing the images in really high quality. An Assembly expert wrote some code that allows you to add this watermark on purpose in the high quality images: http://www.ownedcore.com/forums/world-of-warcraft/world-of-warcraft-general/375573-looking-inside-your-screenshots-4.html#post2491687 We also decoded the content of the watermark and it indeed contains the account information, as mentioned. It is NOT artifacts. Please read the full forum post before posting dis-informative comments. Thank you.