Reference
Guides
Background and troubleshooting for the things that actually go wrong when a form refuses your upload.
Where to start
If a portal has just refused a file and you do not know why, read Why your photo or signature upload was rejected. It works through the causes in the order worth checking.
If you are preparing files for the first time, the requirements reference lists what the major portals ask for, and the submission checklist covers everything worth checking before you press submit.
If you would rather not work through any of it, the tools will resize a photograph or signature to an exact limit in your browser without uploading anything.