destarrow on "Problems with media upload (Png)"

I took some pictures with my Canon, made them transparent and wanted to upload them as always. But the progress bar didn't move at all. After about an hour nothing changed, still no progress. They are about 4 MB big, so I think that isn't the problem. It works perfectly if I upload them in jpg and other images, taken with another camera, could also be uploaded in png with transparent background.
What could be the problem?
Thanks for help