Thanks to LisaS, I can answer this one. I will give you the directions that she gave to me. The pictures actually go into your signature block. If you need help for how to alter that, just ask!
You need the URL for the image which you can get by right-clicking on the picture you want and clicking on Properties. In the box that comes up, select the text next to the "address" attribute (it will start with http:// and end with .jpg) and copy it. Then paste it into the signature block.