It may be problem with image type you upload, size of image or your server.
Can you try couple options? Try uploading small image say 50 KB file size with dimensions 400×300 pixels. If it works then try re-sizing your image to smaller size, for example 800×600 pixels and upload. This way you will define problem more concrete.
It will be more useful if you try to define source of problem. So we can suggest precise solution.