In the top right-hand corner of your page, hover over your name.
A pop-up menu should appear. Click on "Signature"
Find the gif that you are looking for, or upload your own to a site like Imgur.com.
Find the actual URL of the file, making sure that it ends in ".gif"
Copy that URL, and then paste into into the signature box in the Edit Signature page. Put [ IMG ] before it and [ /IMG ] after it. (without the spaces) Double check and make sure it looks something like this: