Animated Gifs require a small amount of extra care when adding to your site. Failing to follow these instructions may result in gifs appearing without their animations.
Adding gifs to a post as featured images
Win: This requires no special treatment
Adding gifs in the body of a post
Loss: This requires you to specify that the image is to appear “Full Size” in the image selection screen.
Why?
WordPress is helpful. By default, when you add a large image to your site it automatically saves several versions of the file in different sizes. Then when a page’s design only requires a small version of the file, it delivers one of the smaller versions. This saves downloading time and makes your site load faster.
Unfortunately, WordPress isn’t smart enough to retain the animation of a gif when it saves smaller versions of the image. Only the original or full size versions of the image will animate.