First you upload you picture a site such as imageshack. Then you grab the Direct URL, you know, copy it. Then you go to the signature editing page and write [ img ] "url here" and end with [ /img ]. There must be no space when writing [img] and the other one. I just can't do it here..
Woa, you never got an answer! Remember, here's lots of people who would like to help you
BY THE WAY - remember there's a limit! 500 x 250 pixels! And that's it!