Basically you can now make your Stories even more obnoxious with the addition of animated stickers from popular GIF platform GIPHY. From Instagram, just click on the GIF option, then search for your desired GIF sticker from the options provided.
For those of us less enamoured by GIFs, Instagram will soon allow you to add photos and videos of any size to a story, in its original dimensions if you choose. According to Instagram, extra space in the Story "will be filled with a custom colour gradient that matches what you’ve shared".
While the GIF option is already available via the latest update, Instagram has yet to reveal when it will add the original photo/video ratio feature. But expect it to be soon since the official announcement is out.