changed game thumbnail source

This is merely a suggestion, not a bugfix:
A lot of games (especially newer ones) don't have a thumbnail picture when taken from the "logo" index in the Twitch API. Conveniently, the API also provides the "box" index for box/poster art, as it can be seen here: http://www.twitch.tv/directory

In my opinion this also fits the overall style of XBMC better than the logo since the box art has basically the same aspect ratio as movie posters.