If you have a .gif, you add it the same way you would add any other image (formatting button in the bar, image bbcode, etc). However, if you are wondering where to find a .gif, try Google and Tumblr; you can also Google how to make a .gif.
Can you link the image? And I believe the image code would be listed with the image on the hosting site, or you can try Code: [img]url[/img] .
