See 'wrjpgcom' and 'rdjpgcom' for embedding text inside the JPEG file. The defect with using them is the moment someone runs it through another graphic processing program and resaves it the text will probably be stripped. But there really isn't a better answer for what you are asking.
There'a also 'exiftool': http://www.sno.phy.queensu.ca/~phil/exiftool/#JPEG Chris
