In any case, go to Imgur.com and upload your Gif from your computer if it's there, copy the direct link it gives you once it's done, then paste it in your signature with the Insert Picture button or by putting image tags around it, like this:
Code:
[IMG]Direct Link here[/IMG]
If you found the Gif from a random website, right click it, click "Copy Image Address" and just skip right to the pasting in your signature part. There should be no reason why your Gif won't play in your signature.
In any case, go to Imgur.com and upload your Gif from your computer if it's there, copy the direct link it gives you once it's done, then paste it in your signature with the Insert Picture button or by putting image tags around it.
I couldn't get Imgur to work as they convert it to some sort of .vgif format that this site is unhappy with so used Giphy instead. There's loads of image hosting websites out there though so surely a few of them will work just fine.