Don't double/triple/quadruple post etc, there is an edit button.
Anyway, you'll need to first
upload the image at a site such as
imageshack and then, from the numerous tags that will appear, highlight the IMG ones, and then use Ctrl+C to copy them.
Then, go to User CP > Edit Signature, and place the cursor ready to type in the text box. Then press ctrl+V to paste the IMG tags and the code, and then click on "Save Signature"
>>;
-Josh