Well to speed up the process, just put this in your sig:
[*css-div=" background-image:url('link here');"]text here [/div]
of course you might want to organize the text, use the center and what not feature to do that, and if you want to specify and certain width and length for the image (it will start from the top left corner and take the width and length from there on), use:
[*css-div=" background-image:url('link here'); width: ...px; height: ...px;"]text here [/div]
instead of the ... put the width and height you need, if you still don't understand, you should check the tutorial Charizard★ mentioned.
oh and remove the *s from the codes.