Need help with your signature? Post your questions here in this thread. Several of our members are experienced with graphics, HTML, and sigs in general, so someone (including me) can probably help you out.
The basics:
If you click on the "Your Details" link up in the top menu bar (the top menu bar is just below the white area and logo at the top of the site), you'll get to the personal setting area. Select "Signature" on the left pane (under profile) and a edit box will appear, much like what you see when posting. If your sig is completely text, you can just paste it in there, click the button to save your changes, and you're done!
If you want any formatting (centering, bold, italics, etc.) or graphics use the buttons (as usual). First of all, graphics will need to be available on the Web somewhere; otherwise, you would be able to see them but we wouldn't!