Taking a screenshot of the YouTube video is somewhat tricky, if you make any mistake you will end up either with YouTube UI or those gradients at the top and bottom in your screenshot. Fortunately, there is a Chrome extension that solves this problem.
While taking a screenshot just pause the YouTube video and click on the Screenshot button to take a high-resolution picture from the video. The extension saves the image in PNG format, JPEG is not supported as of now.
The size of the PNG image will be from 2-5MB so be sure to compress the image before you upload it to any website.