I guess Giphy uses GIFVs, i.e. videos. I filed https://github.com/gavinsharp/SuperStop/issues/10 for adding that functionality to the addon, though I don't think I'll have time to fix it any time soon.

Setting media.autoplay.enabled to false in about:config might be an option for you.

Edit: I updated the addon -- Shift+Escape should now pause videos as well.