I'm not sure about jpeg, but with an algorithm like 'deflate' the encoder is constantly making decisions about whether it's more efficient to represent a series

I'm not sure about jpeg, but with an algorithm like 'deflate' the encoder is constantly making decisions about whether it's more efficient to represent a series of bytes as a literal or as a reference to a previous series. I think a steganographer could take advantage of that by encoding something like a tracking number in the form of the occasional less efficient decision.