I have run into a similar problem a few times and in every case it has been a single image that has stopped the process. In one case, where I really wanted to transfer the image, opening the file in another editor such as Photoshop Elements or Graphic Converter X and then saving it has "fixed" the problem. Identifying the problem image can sometimes be a trick.
If we knew what it was we were doing, it wouldn't be called research, would it?
