Distributors reject artwork constantly, usually for something small — it's not square, it's under 3000 pixels, or it's been compressed into mush by a phone.
If you already have art you're happy with, this is the fix. We square it, size it, and export it properly so the upload goes through.
One honest limit: we can format what you send, but we can't add resolution that isn't there. If your file is a 900-pixel screenshot, no amount of formatting saves it — and rather than deliver something blurry we'll tell you, refund you, and point you at the Cover Artwork service instead.