Bit late but finally got around to rooting around in my last install of WoW. This is Wrath to Cata screens and the marks are clearly there in the standard client version. The marks are there once you sift through a few images and sharpen them to the right amount then you know what to look for and can easily be found in others once you know the pattern used. Pretty much they are aligned in the center portion of screens with ones light on textures the easier ones to pull out, while ones with alot of texturing require a longer look. Think of it like those old stare at posters to pull the image out to understand.
And jacking the quality to 10 to remove them doesn't work, always set my screenshots to the highest setting in the game and they are there.
Probably have some older screens somewhere else but this was my last install of WoW so my older vanilla and BC ones are somewhere else to dig out. I nuked that install when I quit after BC for awhile but kept the screens and decided to keep the last install around on another drive if I get suckered into going back to play.