Is there a simple way to get the default thumbnail from a youtube entry object gdata.youtube.YouTubeVideoEntry?

I tried entry.media.thumbnail, but that gives me four thumbnail objects. Can I always trust that there are four? Can I know which is the default thumbnail that would also appears on the youtube search page? And how would I get that one? Or do I have to alter one of the other ones?

When I know the video_id I use:

http://i4.ytimg.com/vi/{{video_id}}/default.jpg

so, it would also be helpful to get the video_id.

Do I really have to parse one of the url's to get at the video_id ? It seems strange that they don't provide this information directly.