ok usually you have to right click save them as and then upload them to an image host. If you don't know of one you can use www.photobucket.com and sign up. Then after you upload your image from your cpu under the pic is three lines.
Image
http:...
-----------
URL:
http://image url
Tag: <http://image url
Img: [img]http://image url[*/img]
now to make it show up in your signiture just put in the words in the IMG box. Now I am not really good at tutorials becuase I don't have screen shots but hopefully you get the Idea.