Ric,
very simply go to itunes, select one of the tracks from an album with missing artwork in sonos, right click - get info -go to the artwork tab, where you should find the album cover - right click the picture, copy - then paste the pictue into the actual album folder that sonos is reading, finally rename the picture folder.jpg.
That should do it, however the problem is that sonos will already be reading the embedded files anyway, so there is probably a problem with that particular image, most likely it is too large - in that case just find a smaller image on the internet (600 x 600 works fine and still looks ok on a monitor) and replace the image in itunes for the entire album
itunes - highlight entire album - clear downloaded artwork - get info - copy and paste new picture into artwork box (and you might as well save the picture in the folder as well as above.
hope that helps
Bill