Comment by jfengel
I do like the idea of storing it steganographically, which also serves as a watermark.
But it requires a ton of redundancy and error correction, perhaps enough to survive a few rounds of not-too-lossy reencoding. I dunno how much bandwidth is available before it damages the image.
I wonder how practical it'd be to stratanagraphically hide a QR code in an image and it be retained through rounds of JPEG compression. It could be represented with a single bit per pixel and would inherently have some resistance to corruption.
The real limitation would be bandwidth.