Its real easy, i figured it out, so I know you can.
save the picture to your computer. click home (at top) on xg.com, then user CP (on right), edit sig( on left), you will see a thing that says add sig and browse from computer, do it. save it (at bottom, important) and then click the thing to the right of your picture that says somthing about add to sig.. you will see some letters appear in the sig box. thats it, just post and see.