ok first off.. look above at the blue bar, click where it says "file manager". in the file manager window click browse and browse for the image on your computer, select it and click open. then click upload and wait until the page refreshes. once it refreshes you will see "q.gif".
click "q.gif" and at the top of your browser copy the URL to where the image is located.
now go back to the blue bar at the top and click "user cp". on the left, click where it says "edit signature". now type the tag "[img]" and then after that paste the URL to the image and close the tag by typing "[/img]". it should look like this:
click save and you should now have a signature!