Upload & Sell: Off
Digital embedded watermarking. Encrypted EXIF data. There are ways of marking something as yours that can't be erased in 5 minutes by some schmuck in Photoshop.
Unfortunately none of those methods is easy to detect at the other end. So even if you have encrypted EXIF (which I can strip in 10 seconds), someone must download the photo, then decrypt the encrypted EXIF just to determine it's yours. I doubt it.
Steganography holds the best promise of undetectable tamper proof watermarking, but it serves only forensic purposes, not practical ones.