I've got a thread on this going on at Symbiostock.org, too. If the Christine here is the same as the one that replied there, you can focus on that thread instead of this one. However, if you're a different Christine, here's some info-- I've had a variety of sizes show this same problem, from over 5000px to only 640px, the size seems to make no difference. Most of my images are a hair over 3000. I've also had many of each size display correctly, even the 640 size, correctly showing 640 as full size. Whatever the cause, I need a fix.
Perhaps someone knows where the data is stored for the pricing tables for each image, so I could go and correct the errant ones manually?