It's easy Cetti, find yourself a nice animated gif by doing a search on the net, right click on the pic, save it to 'my pictures' or 'desktop', go to your AB beta profile and click where it says browse (for your avatar) direct it to where you saved your picture and upload it. It will upload if the file name ends in .gif and it's less than 10kb in size.
I always have files of animated Christmas gifs, Halloween gifs, birthday gifs, smiley gifs etc. for inserting in my emails. I just do a search for either gifs or animated pictures and save the ones I like to my computer.