Welcome to the AmpJuke forum and thanks for posting.
I'm a little bit unsure if I can pinpoint what your problem seems to be.
I do not have difficulties using the Bing-thing: It suggests album covers and artist/performer images like it's supposed to when I click "Lookup a new image" and/or "Lookup a new cover". Besides the Bing-stuff I also get suggestions from last.fm, as expected.
A dumb suggestion would be that you've hit the limit when asking Bing...
...or - an even dumber suggestion - punched in a wrong API-key for Bing..
You're absolutely right that there's not anything logged in relation to what happens between AmpJuke and Bing.
However, the "bing_lib.php" file might be a good place to start to at least start to "echo" stuff to the screen within various places...
Hope some of this helps.
If not, please post an update (well...feel free to do so in any case).