It worked for me using (Windows 10) "K": "mpv:screenshot", but you may want to tinker with settings in your mpv.conf:
–screenshot-jpeg-quality=80 ## Default=90
–screenshot-png-compression=4 ## Default=7
–screenshot-directory=~~desktop/Screenshots
–screenshot-template=PlexHTPC-%wH,%wM,%wS
Otherwise, depending on your processor, higher quality settings may interfere with playback. I actually use "K": {"short": "mpv:screenshot", "long": "mpv:screenshot-to-file D:/PlexHTPC.png"}, in my inputmap so that a short press gives me a JPG and a long press gives me a PNG, but I had trouble feeding the screenshot-to-file the same desktop destination as the JPGs…
The screenshot-template setting above names the files based on the playback timing when the snapshot was taken.
