if the track is audio only, it'll look in a cache of albumart, if not,
request the big google in the sky for an image with some search terms
guessed from id3/metadata/filename and then use that. tak'es the first
thing found. not very smart, but pretty damned good most of the time.
these thumbnails are generated in the bg by rage_thumb and will appear
once generated (or already found cached). it'll generate new thumbs if
modified dates are equal or newer on tthe src vs the thumbnail cache.
the caches store 160xN thumbs (keep aspect) with 70% lossy jpeg
quality in an eet file with one thumb per 10 second interval. so it
has limited accuracy.