The intent is to display more clearly what the Live Photo image is composed of, particularly when the frame displaying the content is taken from a blurry or unclear segment of the Live Photo. Now, users can cycle through any frame from a Live Photo in Motion Stills, choose a new image, and export it back out to the iOS Photos app with the new frame. This exporting process keeps the Live Photo in Apple's custom 3D touch looping format and not the GIF format of Motion Stills.
As The Verge pointed out, there is a catch in the basic function of Live Photos that could result in a customized frame appearing lower-resolution in the Photos app.
There is a catch. For space reasons, Live Photos only save the primary frame in full 12-megapixel clarity, with the other frames handled as lower-resolution video; choosing one of these frames will therefore result in a photo with less detail than the original. But since the vast majority of Live Photo viewing probably happens on phones, this may not be a major issue for you.Other than the new frame feature, Motion Stills as a whole lets users turn Live Photos into easily shareable GIFs. The app also uses Google's video stabilization technology to smooth out jittery images and freeze backgrounds, giving an overall improvement on Apple's basic Live Photos. Motion Stills can be downloaded for free from the iOS App Store. [Direct Link]