It will come up automatically after you've saved it. Right, the way you are doing it is inserting the picture from your computer, correct?
Have you done this? You've gone to user CP, edit signature, scroll down and underneath the signature picture column there should be an "Choose File" under option 2, select your picture from your computer and okay it. Then you should have a picture appear above the option, click the "Insert Signature Picture" and you [SIGPC][/SIGPIC] will come up but once you preview it it should in theory be there, once you've done that you've scrolled to the bottom and clicked "Save Signature".