Use screenshot of GitHub YouTube channel when not logged in

Swaps out the screenshot of the GitHub YouTube channel page since current is logged in under an account.

The page looks markedly different now as well after their redo.

Went with _Videos_ instead of the default _Home_ tab, former feels like it would match more. 

Could (should?) have gone with adding in the [GitHubGuides channel](http://www.youtube.com/user/GitHubGuides)