HI , you can select the key photos / still images on your iphone with IOS 11 (per below) and there is link with instructions above for doing this on a mac with high Sierra (11.13) installed (sadly new OS does not run on my old 2012 imac
Here is the link / instructions for doing on iphone (it is similar process to burst photos pick the image you like and turn on or off)
Want to use a different key photo than the one you see in your photo library? In iOS 11 and later, it's no problem. Choose from the range of frames that make up your Live Photo.
- Open the Live Photo.
- Tap Edit.
- Move the slider to change the frame.
- Release your finger, then tap Make Key Photo.
- Tap Done.
You can also toggle the Live Photo effect off when you tap Edit >
. If you change your mind, just tap
again in Edit.