When you find a picture you want to put into your signature, here is what you do:
First you need the URL address of the picture. To find the URL of an image, right-click on it. Then click on "properties". A box will pop up, and about halfway down it will show the URL address. You can just highlight it, right-click on it, and click "copy". Then go into your profile by clicking "my profile" at the top of this page. Scroll down to the signature box, and type in . Put the URL address of the picture you chose right smack in-between .
That should do it.