This is in the wrong forum. ^^; This kind of question should go in the Community Questions & Feedback forum. :3
To answer your question, just place the url of the piccy you want into the area where you edit your signature. Then highlight the url and click the little button that either looks like a picture frame or some mountains. If you can't find the button, just wrap the url with the tag and click Save Signature. ^^ I'll close this now. ^.~
*closes*
~Kelsey