I think some of them have been leaked through someone at Topps. Some of the images that have surfaced are quite similar to images that have been shown on the upcoming Topps ROTS cards.
That's a pretty good guess. Sounds plausible. That Dooku/Anakin faceoff leaked first and the Topps card showed up just after.
The guy that leaks them onto MF.com, "Spoilerific" (if the name is actually a person) posts images all of the same size, like they came off an image library system similar to one we had at Disney. There was a pooled image library database of 1000's of stills from the features that you could then request as a high-res image. The db was accessible through all on consumer products, product development, and select licensees.
Must be very frustrating for folks working on the movie and hoping to save some surprises for May.
Edit: Not that I care if they're frustrated, I want to see every frame I can, asap.