Ok... So as attachments, animated GIFs will not work. That is a security function of the forum and I believe, there is now way around it.
Easiest method is to upload it to an image hosting site and use the HTML code to insert it into your post. There is a button, second from the left bottom row, that will insert the code. You just need to paste the link. Will look something like this:
[img]Your Image hosting link here[/img]