GIFs work in much the same was as any image on a site like this; you'll just need to add a HTML code before and after the URL link (see below).
For example: (remove the spaces for this to work)
[img ] http://i.imgur.com/Ze7HH.jpg [/img ]
The 'blue' is the HTML code (N.B. that there is one before and one after) and the link is the URL link copy&pasted directly from the picture. Together this makes this:
Hope this helps.