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 images will often present too much of a challenge. We recommend re-sizing images (the Cirrus screen-width is set to 700px, but will take a larger image) 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.