Taking a screencap of a YouTube video isn't as easy as you might think. Even if you full-screen a clip, chances are if you time it wrong, you'll end up with part of the YouTube UI in the shot, or those annoying gradients at the top and bottom. Fortunately for Chrome users, there's an extension that will save you a lot of trouble.
The aptly named "Screenshot YouTube" is a lightweight Chrome extension that adds a "Screenshot" button to YouTube videos, near where the settings cog is found (see image above).
Rather than capturing the screen, the extension actually copies directly from the video stream, taking advantage of HTML5's canvas functionality. That way, you get a nice, high resolution image, without any of the UI guff.
Once the shot is taken, it'll be automatically saved to your Downloads folder as a PNG, ready to use in your image editor of choice. The extension's only downside is that it doesn't support JPEG, so be sure to delete those PNGs once you're done, as they can vary from 2-5MB in size.
Screenshot YouTube [Chrome Web Store]