Quite late on the reply here, but I just solved this problem for myself after having it a while.

It occured every time I chose "Save Image As..." from within Firefox (many versions/revisions). It only happened with Firefox; browsing the folder with explorer, etc. didn't cause any irregularities. I saw the suggestion about file associations and thought instead to try file names. I renamed a few of my files (less than half a dozen at the front of a folder of 100+) and every thing seems to be working perfectly again.

Hope this works for someone else.