When making a post, you click on the window icon above where you enter the text. It's the 4th one from the right-hand side.
Get gif url Click the portrait image of the ocean view Enter gif url Or Use img tags HTML: [img]url[/img]
If you upload the file using the button below (beside the post reply button) click full image and it should come up Or you can upload your gif to a hosting website like imgur (or just copy the link of an existing one) and click the picture button to the left of the emote button. It should work just like regular images...
Also if you are viewing a gif in another website I'm sure you can just drag it to the post box and it should work.