Go to your User CP and look on the left-hand column until you see the 'Settings & Options' subheading. Then click 'Edit Signature'.
If you want to use pictures in your sig, use the
You must be registered for see images
button and copy-paste the URL of the image you want.
To get the URL, right-click on the image you want to use and choose either 'View Image' (and copy the URL manually from the address bar on your internet browser), or click 'Copy Image Location'.
Bear in mind, the image needs to be scaled at about 600x300. Any larger, and the picture is too big.
