Currently the user can decide between showing artwork for "Now Playing" or "Selected Tracks". Awesome!
Now... what if the following behaviour was implemented?
When "Now Playing" is chosen:
If a track is playing, show artwork for that track. If nothing is playing (that is, for instance, after clicking "Stop" but not after clicking "Pause"), show artwork for selection. Similar to iTunes behaviour.
When "Selected Tracks" is chosen:
Show artwork for selection. Duh.
Peace.