GIFS are the same thing as uploading pictures to the forums All you got to do is copy the link of the GIF, then go to your signature and click the image button then paste the link and you are done. (Some link won't let the picture show and it will be broken)
Click your name on the top right it will look like this but instead your name Then hit Signature Then this will pop up: And if you want to add a video like YouTube for example you hit: Once that is open make sure you copied the YouTube video URL so for example when I paste in there it will look like this: Then click Embed and click: Then you will be done also you can align it if you want or anything else you want to do.
A form of computer image that moves as an animation, because it consists of frames, like a movie with no sound.
Do the same steps but go to the this part: And put this in there: http://29.media.tumblr.com/tumblr_ljvo2kwpiN1qj6u8oo1_500.gif