A quick google will give you loads of options for downloading Youtube videos. As for stills, just pause the video in the appropriate place and then take a Screenshot.
My suggestion is MacX YouTube Downloader. It can download videos from YouTube in a few minutes. For creating image issue, it has a "Snapshot" button on the interface so that you can capture any picture you like from the original video. The most important thing is it is freeware, fantastic, right
Hope this helps