Sign on white paper and photograph it. Those are the numbers the form is checking, and getting either of them wrong is what an upload rejects for. This does both at once: the photograph is cut to 140 × 60 pixels and then compressed until it is under 20 KB — and no further, because quality is only worth giving away as far as the limit demands.
Make it now →Free, in your browser. Your photograph is deleted the moment the job finishes.
Almost always one of three things: the picture is the wrong shape, the file is over the limit, or the background is not plain. A form checks the first two before it looks at anything else, which is why "invalid photo" so often means "a hundred kilobytes too big".