Just as a post sometimes says (short) or (long), if the post only has a GIF in it, it should say (GIF).

I realize that some of the time, if somebody responds with only a GIF, the common CB etiquette is to leave the subject line blank, but it’s not a perfect system. This would make it more easily identifiable so I can know not to click your post, cause most of you are super awful at responding with GIFs but think you need to do it all the time.