The problem could be image size, a corrupted file, or a browser issue.
If the Cirrus system won't upload an image, it's usually a matter of scale. Cirrus will re-size your images for you, but extremely large, very high-quality mages will often present too much of a challenge. We recommend sizing images to a maximum width of 700dpi (pixels per inch) before attempting to upload them.
Occasionally, the system will reject a file despite it being an acceptable size and accepted format. If your file seems okay otherwise, try saving a .jpeg as a .png (or vice versa), and see if it will upload in the new format. If it still won't upload, the image might be corrupted.
Finally, check that your browser is up to date. Some older versions of the browsers (especially Chrome) cause problems including difficulty uploading images.