[mxuribe]

While I haven't read Amazon's terms of use as to whether this is ok from a policy perspective, most back-up services don't check for "files within pictures". In fact, the hiding of data within imagery is called Steganography: https://en.wikipedia.org/wiki/Steganography

Although the intent with Steganography is (weak) cryptography, the method should work for your purposes; only way to know is to test it! Admittedly until you've tested this thoroughly, I would not RELY on this for anything other than a fun test. Enjoy! :-)