is it really a JPG
My customer had similar problems with jpg's and it drove me crazy. I would check the file and make sure it was good.. save it and upload it with no problem. Finally it was the jpg's they were creating. There is a problem with images made with certain HP and Kodak cameras where the jpgs wouldn't upload, in fact one of my companies that provide batch processing of files for us was the one that made us aware of the problem. They can out with a fix for the program we use for batch processing and it solved all problems. We had our customer buy their program and now this problem is solved. We chased this problem for almost a month so I hope this helps.