You need to upload the picture to an hosting site - like photobucket or image shack etc. Then click on "My Controls" which is at the top of this forum, then click on "Edit Signature" down the left side - once you've done this, you'll see an empty box appear - this is where you put your signature.
If you want a picture you need to put this - Obviously you need to take out the spaces

[ i m g ]then the url of the picture you've uploaded goes here[ / i m g ]
Then click "Update my signature" and you're done!