An invisible way to hide a signature comes to mind. Until the ownership comes into question no one will be aware that the image itself contains all proof needed

An invisible way to hide a signature comes to mind. Until the ownership comes into question no one will be aware that the image itself contains all proof needed. You could do something like Eurion and hide a pattern that only copiers can identify. Or do the same as color printers and encode hardware and user information as a pattern. DOOM 2016 hid satanic images in its sound files instead of other images, of course that was only an easter egg and not really useful outside of confirming that computer games are evil.