The iPhone 6s and above can take what are called Live Photos. They have a key frame photo but also show you a second or two on either side of that key frame photo. The following steps will allow you to extract an image that is NOT the key frame, e.g. one taken before or after the key frame.
Live Photos are actually two files – a single JPG of the key frame, and a movie (.MOV) file. As of the time of this writing, the Movie file is taken at 1080p while the JPG can be much higher resolution. It’s possible Apple will raise this resolution at some time in the future.
Following the steps described here will give you an image that is 1440×1080, not the high resolution size of the JPG. You can’t extract a higher resolution image because it simply does not exist.
Import Live Photo into Photos
From the File menu choose Export – Export Unmodified Original for 1 Photo
Choose Folder and Select Export Originals
You’ll now have two files: the JPG of the key frame that the iPhone already showed you, and the associated movie file that shows the animation before and after the key frame.
Open the Movie file in QuickTime
Drag the scrubber till you get to the frame you like
From the Edit Menu Choose Copy
Hit Command-N for New and the copied image will open in Preview.
Alternatively you can use the menus (like an animal) and choose File, New from Clipboard
Save the Image
Note the Size Difference
Our extracted image is 1440×1080 because that’s the resolution of the video that creates the live effect. The original JPG was 4032 x 3024.
You simply cannot get the high resolution image of other frames because they don’t exist.