Currently the application uses amazon to search for images of CDs.
It's a pain for friends of Heavy Metal because many underground bands
are not available on amazon.
An Image search on Google most of the time has much better results for
this kind of music. Only mainstream music is available on Amazon. The
bad thing about it, that rhythmbox often fetches a totally wrong
image, because it couldn't find the right thing.
Maybe a better, if not more convoluted method, would be to get MusicBrainz to add support for the bands' images, and then Rhythmbox to use MBz in addition. MBz culls from a variety of sources (including Amazon, but also others) and adding support for it would be more widely noticeable.
I can't speak for the developers, but I can imagine the reliability of Google Images for cover art would be very low.