A fast, easy and free way to get full screen images for your YouTube cover images.
To create thumbnails for your YouTube videos, you may sometimes want to grab screen shots of your video game play. One way to do this is to watch your video in full screen mode with the YouTube media player. Just click on that bracketed  pair in the lower right hand corner.
To avoid having the title and play controls show while you capture, move the mouse cursor off the screen and onto your other monitor.
Have Windows Snippet Tool running and set to capture in Region mode. Click the Capture button and the screen will be ready as a still to be enclosed with your mouse. Draw a rectangular boundary as close to the edges of the screen as possible. You can then save as a JPEG file to disk or copy to the clipboard to be transferred to a photo editing software.
I use this method with Adobe Photoshop CC to quickly create layered thumbnails with YouTube. Its a fast, quick, and easy way to generate good looking thumbnails.