Sounds to me like some weird IIS permissions mess up. I have seen this once or twice over the years but it is never reproduceable by anyone.
Make sure your data directory of classifieds is set to every level READ WRITE EXECUTE. Try installing Imagemagick on the server is you can instead of using GD2.
The key here is the only info I have ever been given by the one or two people is this happens when PHP whacks out on the server maybe from a big file upload.