Um
Get a Gif
Make it 75x75
???
Profit
How helpful! 10/10 you should write a book.
When you mean animated, you mean a .gif file. In order to upload an avatar image, you must either have the link of the image, or have the image saved onto your computer. To save an image's website directory, just right-click on it and click 'Copy Image Location'. If you want to save said image, right-click and click "save as" under what file directory you choose.
Once you have gotten that done, go under 'Profile', and then 'Forum Profile Information' in the forum. Click on the 'browse' button after clicking 'I will upload my own picture'.
Now, you can either paste the image's website directory by pressing ctrl+v, or you could select the folder the image you saved is under. Keep in mind that the image must be in or under the dimensions 75x75.
After you've chosen it, click 'Change Profile' and you're set.