You can find someone to make you one, or you could make one yourself. Then upload it to a image hosting site, like imageshack or photoshop. Then copy the link and go to "edit profile" then change the avatar URL.
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"Rescue the president, Rescue the hostages, disarm the terrorists..."
"Rescue the president, Rescue the hostages, disarm the terrorists..."
"Rescue the terrorists, disarm the president, and kill the hostages..."
"Disarm the hostages
You'll need to use the html code for images to put your signature image in. You can find code for that and most other common functions on the right side of the window when you are posting. You'll want the code for "image."
Usually, when you upload a picture, on the page there will be links. Copy the one that says something like direct layout or something, then paste it into the avatar box.