Most photographers sign their prints on the back. Some sign in front, in a margin that is left on the photo paper. No artist would sign the mat board that frames the print, because if the mat were replaced, the signature would be lost. If the the paper is fiber based paper, many artists use pencil. If it is RC paper, it is difficult for pencil to stick, so indelible ink pens could be used. If it is an edition, that information should also be with the signature. Each artist chooses his or her own method, and then sticks with it.
For all your subscription questions, go to the Book_Arts-L FAQ and Archive.
See http://www.philobiblon.com for full information