How close to 5 MB are we talking? Sometimes an upload won't quite fit if it is right around the limit (5MB per upload on a Free account). Could you save it at a just-slightly smaller file size and try that? Next possibility, if your entire account Quota is full, that can also cause it to fail. This can be checked on your personal account page. If these thoughts don't resolve it, we will pursue some other usual troubleshooting steps.