Sure.. when you call [man]imagegif/man or [man]imagejpeg/man, just send the second parameter, and it will output it to a file instead of to the browser 🙂 (no screaming!) Then you just reference the image as you normally would 🙂 (no screaming!)
Now, you'd need to set up a CronJob (google it) to update the image every once in a while.
imagegif(image handeler, 'my_signature_image.gif');