Easy, get the link of the picture that you want to use (If you have one on your pc, you can upload it to a picture hosting website and copy the link) and then, copy and paste this into your signature: [img]LINK OF THE PICTURE HERE[/img]
Paste the link of your picture where it says: LINK OF THE PICTURE HERE
You can mess around with it too, such as specifying the width and height of the pic, just replace the first [img] with [img width=!height=!]
Replace the ! with the desired height/width in pixels.
Edit: Vespian got to it first XD