Click on on the top of the page.
Now click on My SignatureNow where it says "Modify your signature" add the link of your pictures that you have hosted. I use Imageshack. Here's how:
On imageshack's homepage (
www.imageshack.us), you can select the size of the pic you want. I usually choose the originlal size. If it's too big, you can choose a smaller size. Here's how to do it. Look at the red text: