I figured people could figure this out by now..
Ok, when you click edit sig, above the white box where you would type in text, you'll see a button that says "img" when you click img it will bring up a box. In that box you type in the url of the image.
If the image is on you're computer and NOT online, you'll have to upload it, just like I did with mine. After you've uploaded it, right click on the image, get the url of the image, and post it in the img box.