You would need the following programs:
The Infinity Engine Explorer; available from the Ironworks website (correct me if I'm wrong)
The GIF animator (trial version available from
www.ulead.com)
What you would then do, is go through the inf. explorer and find an animation you like, export it to a folder e.g. c:\animations. MAKE SURE TO SAVE THE FILE AS A GIF ANIMATION! Then open the gif animator, load the file and 'loop' the animation. Next, save the file, and find a site to upload the image onto. (I used angelfire.com)
When you have uploaded the animation it is ready for use in your message!
Just type [img=link to image] and there you have it!
Hope this helps!