Step 1 : Click this link YouTube Thumbnail Image Generator.
Step 2 : Go to www.YouTube.com, Search your video.
Step 3 : Copy the URL of the video ( Example: https://www.youtube.com/watch?v=RK1K2bCg4J8 ).
Step 4 : Paste that URL in the given textbox and then click the "Get YouTube Thumbnail" button.
Step 5 : Choose your Thumbnail size and Download it.