Are you using OSX? If you are then it's pretty trivial. You will need a graphic tablet and pen like the Wacom Graphire. You can create a transparent layer in Photoshop and just write your signature, save it for use later on when you can paste on whichever photo you want.
You have the pad, so that makes it easy. Just like mentioned before, but I would just open a new document to a width of an inch or so (whatever suits you, can always scale it down), with a transparent background, then do your signature and save it,then can layer it whenever.
