At the risk of "ruining" it for some users, Amazon doesn't appear to validate nor re-compress uploaded images. I uploaded some bits from /dev/random as a .png, and it counted towards photo storage, and I got the same bits back re-downloading the test file. So you don't even have to resort to stuffing metadata or steganography.

For the record, I don't exploit this loophole and only tested to satisfy my curiosity. At $60/yr. it's already very cheap for unlimited -- it's my tertiary backup so I'll accept the risk that "unlimited" is qualified -- generic file storage.