you can add animated GIFs from giphy.com by referencing their ID in comments, like this:
"value": "Look at my comment!!!! $id:26FPGsG2NZL1QzAFW$"
You can get the ID from giphy's website, looking at the URL of the image.
E.g. http://i.giphy.com/26FPGsG2NZL1QzAFW.gif has ID 26FPGsG2NZL1QzAFW
Hope that helps.