What browser do you use? I'm using chrome. Not that other than chrome you can't do it, it just more simple when I use that in my tablet. Nevertheless, any kind of browser didn't matter, ONLY when u use opera mini, the gif won't move.
When I found a funny gif in google, I just view the image and copy the url link. Then when I want to post it in forum, I'm gonna look for that tiny mountain pic or image hosting link. Click it then paste my link gif's and boom!!
Paste it exactly on the http so when you do it, you will get one http link instead of http http link.