We've just launched a new feature - while listening to a track we'll automatically provide you with a compilation of artist images; the background of your player will be automatically adopted! On the right you can see an example for Radiohead.
We're using Flickr as main source for the images and only CC - licenced images will be used (with the proper licence).
You can disable this feature in the preferences (Section "Design").
Image Sources: 1 2