This free to use online tool helps you generate screenshots at any time for any Youtube video. Just copy paste the youtube video link in the above field and hit "Generate", This will present you with a youtube video preview. Simply select the time for which you want to take the screenshot. Hit the button "Take Screenshot", A screenshot will be generated at the bottom of the page.
If you are on a desktop, right-click and select "save as" on the image.
If you are on mobile press for a while on the image and select "Save as"