A Live Photo is a pair of a high resolution still frame and a video clip.
If you transfer a Live Photo to a computer, you will receive at least two files, the still frame and the video component. A simple way to turn the Live Photo into a still frame is to discard the video component of the pair on a computer, but keep the still frame, then to delete the Live Photo from the iPhone and to transfer the saved still frame back.
To separate the pair without a computer, send the Live Photo to yourself by Mail. Select the Live Photo in Photos, tap ⇧ , then select to share by Mail. Mail will only send the still frame. Convince yourself, that the Mail attachment is readable, then delete the still image from Photos and save the attachment back to Photos.