I just went to google images and searched the gif I was looking for. After I checked it was animated I copied and pasted, but I copied the image location, and pasted into the image thing.
Probably because the picture you posted is a jpeg. Note the ".jpg" at the end of the file name. GIF's have ".gif" at the end, and not all of them are animated. But just adding them like a normal picture is enough to show the animation on this forum - just click the tree button in the editing area and type the picture's url.