2 x 2 inch (51 x 51 mm), square. 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 600 × 600 pixels and then compressed until it is under 240 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".