Can you try converting the image to another type? If I had to guess, the site probably doesn't handle newer image encoding. The oldest versions of .jpg files may work, although I've been told some jpg files do not (maybe there's a difference between old and new jpg files). If you can, try loading the image into a different image viewer (such as MS Paint, then save it to a different file format, and try loading that).

If all else fails, please email me a copy of the problem file (I'll message you with my email address). I wouldn't mind getting to the bottom of this and knowing if there's a workaround.

Thanks
Dirk