Its same as you use any pic for your avatar, the process you used for setting your current avatar use the same process to select the gif avy. or if any image is on net then right click the image > View image > copy the url of image > paste it in your change avatar pic bar in your user cp > save changes.....booyah you have gif avatar, i hope i'm not too confusing
Click on your avatar, it will direct you to the page where the edit signature option is. In" My settings " panel you'll see " Edit Signature ".
Click on the " Insert Image " option and copy paste the image url. Click ok and then save signature