BRO. You're back. :)
1. Take your favorite .gif off any website (the ones I use come from Tumblr. If you like a certain anime/show, just type the name of it as a tag)
2. Edit your .gif to suit the avatar limitations. I use this website to shrink and edit my .gifs into squares.
3. After saving the editied .gif on your local computer, take that .gif and upload it on this editing website so you can optimize (and reduce the size) it in the >Edit menu. Your .gif needs to be 51200 bytes, or 50 in Kilobytes. As a result of optimizing your .gif, the quality will go down, so be cautious.
I hope that's understandable :)