Code review comment for lp:~jamesh/camera-app/pre-cache-video-thumbnails

Functionally, it's great and achieves what the description promises with a very minimal cost. Very nice improvement indeed!
Code wise, only a couple of details:
- it would be nice to unload the Image once the thumbnail has been loaded; although it is a very small save of a 32x32 image
- the Image {}'s id could be more explicit than just 'thumbnail', for example 'preCachingThumbnail' or something
- also a small comment next to the Image explaining its purpose and specifically why a sourceSize of 32x32 suffices to provide all the benefits would be extra useful

Thanks a lot!