Increasingly, YouTube is proven to be an excellent source of images. More and more videos are uploaded in 1080p HD (you can change the resolution in the settings menu at the bottom right of the video) which creates very good stills when you pause the video and make a screen shot.
To get the best results, try to size the YouTube window in such a way that it is around 1080 pixels wide.
If you want more control over what images to take, you can download the YouTube video to your machine (instructions here) and load the video into a video editing program such iMovie. Here you can move the playhead frame by frame to get the perfect shot.
Because of video encoding, you will see that images get blurrier when there is a lot of movement in the scene. If you want to show action, go ahead and grab the screen shot. If you want a crisper picture wait a few 1/24th of seconds until everything is calmed down again.
An obvious point: make sure that you have the rights to use the content.