There are two possibilities :
1- Click on "go advanced" below (it's located next to the post button), then click on the clip image (
). After that, browse for the gif file and click upload. When that's dont click on "submit reply and you're done.
2- We all prefer this method. Go to any image hosting site :
http://imageshack.us or
http://photobucket.com , make an account there and upload the gif. After that, they will show you some links for the image, such as (direct link, IMG link...). You can copy the direct link, paste it here and put the image tags around it. It should look like this.
.
Or you can simply copy the IMG link and paste it without doing anything else.
Hope this helps.