It turns out that there's a lot more information captured in a World of Warcraft screenshot than most players would ever think to look for.
A user at OwnedCore unearthed the invisible watermarks, which then spilled forth onto Reddit. The gist is that a screenshot taken inside World of Warcraft with its screenshot function includes a certain amount of embedded information in a repeating pattern. The information includes an account ID, realm (server) information, and a timestamp. Screenshots taken using a third-party tool (like Steam or Fraps) do not appear to contain the watermark.
Some users, like the original poster at OwnedCore, are concerned about the privacy ramifications of watermark information floating around the internet. While no password or private data are embedded in the image, according to the original poster, a particularly dedicated person could look at the images and link several alts together, or determine a pattern of individual user behavior. The information in the images is more useful to Blizzard, who could theoretically use the data in the screenshots to trace TOS-breaking behavior or leaks of material under an NDA back to the source account.
Kotaku asked Blizzard for a comment but as of yet have had no response. We'll update if we hear back.
In the meantime, if you're curious about your own screenshots, finding the watermark is fairly straightforward. There are directions available for looking at images using IrfanView, a free image editing program, to sharpen a screenshot to the point where the watermark is visible as an artifact. For best results, the image needs to be as close to white as possible. Using this base image, a few minutes tweaking sharpness and light filters in Photoshop yielded some surprisingly cool-looking variations. (For best results, a black layer set to "hard mix" will make a watermark underneath it pop.)