Hi Gabrielle,
If you don’t have a stack that lets you select the “poster” image from youtube, another option might be to use Topbox (or similar lightbox) and have the still frame image of your choosing (via a screen capture or frame capture) in an image stack set to be the “trigger” stack for Topbox, which would open up the youtube video.
Topbox works very well for youtube or vimeo. But of course then the video is opening in a lightbox as opposed to just existing on the page in a youtube sack.
Also as Mathew said the quality of the screen capture will depend on the resolution of the original video uploaded to youtube or vimeo. The best quality available of the video on your page is 480p (which you can select in the youtube’s player “settings”), and while not hi-res, it doesn’t look too bad at the size of the youtube stack on your page. This way visitors who have to choose a lower res version on youtube because of slow internet speed will still see the still frame at the higher res.