Let's do it in steps. You do it the same way as you add any kind of picture.
- Copy the images URL. This can be done by right clicking the image and selecting 'Copy Image URL'. For it to be animated, the image type must be .gif.
- Press the little picture icon on the toolbar above the answer/question box. This will give you a little pop-up that will ask you to paste the URL. '
http://', the beginning of a site's URL, will already be in the box, so erase it if you are pasting the URL as a whole. Once done, click 'OK'.
- You have your image. Don't erase any of the coding displayed, as it will not show an image if you do so. The only thing you may change is 'enter image description here', but it doesn't matter at all.
The code will be replaced with the image when you post it.
4. Post your Q/A. If you have done it correctly, you'll have your image. If it was a working .gif file, it will also be animated.
An example here: a Forretress sprite.