When you open a video in YouTube, the flash player plays the flv file. If you want the high definition version, click on the HD button presented at the bottom of the flash player in your browser. The HD version will be loaded up. The HD version is more in size and is better in quality too.
In windows you can search for keyboard from start menu and open keyboard settings. Or you can go to Settings > Accessibility > Keyboard. In the keyboard settings window, toggle the switch that says "Use the Print screen button to open screen snipping". Hereafter whenever you press print screen button, you will be able to make a selection on the screen for a screenshot.