After that copy the DIRECT URL link to the image. They usually end it .png or .jpg ETC
After that go over to your signature page and use the Image feature in the text box to add your image
You can see it done here: http://www.mpgh.net/forum/showthread.php?t=559173
After that you're good to go.
FYI, the forum uses the same feature not only on the signature page but on the regular forums as well. So next time when you upload an image do the same method as you would uploading your sig picture.