We're talking about images, so if you take the pixels for example they're just {0,...,255}^n, integers. You can if you have the original file, but then what's

We're talking about images, so if you take the pixels for example they're just {0,...,255}^n, integers. You can if you have the original file, but then what's the point? Otherwise, take image a=original+f1, image b=original+f2. Then if you e.g. average them you still have c=original+f1/2+f2/2 -- so both signatures are kept (theoretically if either you have a large number of sources you could average the signals out of existence, or perhaps the system isn't robust and can't detect multiple sigs). This assumes the introduced signatures are indistinguishable from the source (quite reasonable).