Glad you got it sorted. For the sake of anybody else who sees this...
Either upload a gif from your computer/ phone as you would a normal picture. As long as it has saved properly to your computer as a gif it should upload as one too.
Or find a gif online and get the web link for that gif (sites like giphy are helpful for that) then type...
And voila. You've successfully inserted a gif with super hacker code skills.
edit: okay so my computer didn't show me any of the loads of previous responses. Sorry for the pointless thread bump XD