First find and click the "insert image" button on the forum, which is located above the text box where you write your posts. If you have your gif saved to a photobucket or something, copy the direct link and paste it into "insert image" address box that pops up. If the gif is online somewhere and you aren't going to save it and upload it, then right click the gif and click "copy image url". Paste that into the "insert image" address box and you're good to go. Get that down and you'll be posting gifs in no time, like a real pro.