The beautification of my MP3 collection the other day paid off nicely for me, but I discovered a few tracks and albums for which the cover art didn't show up on the Sonos controller for iPhone/iPod Touch.
I know the image is embedded into the tracks, because iTunes shows the art nicely enough
as well as the Sonos desktop:
For a moment I thought I had some network glitch or other that was causing the iPhone to not extract the cover, but it turns out that the reason the controllers don't display the image is that the size of the image embedded into the tracks appears to be too large. In the example above, I wasn't paying attention and dragged a 211 Kb image into each track. (When you have masses of free space on a NAS you don't pay attention to minuscule details like wasting a meg or two. ;-) )
The Sonos controller Apps quite reasonably cannot handle that volume of data in the cover art image (and there is absolutely no reason to do so), so they simply ignore the art.
Instead of dragging an image directly into your music tracks, I recommend you drag it into an image resizing program instead (I use ImageWell for this because it shows me image and file sizes at a glance), resize the cover art image to some reasonable size (something along the lines of max 300x300 pixels, keeping the size of the file well under 200Kb), and only then drag the result into your tracks. (More hints.) Note that the number of bytes is what causes the problem, not the image's resolution -- a 600x600 Jpeg of size 95 Kb works fine.
So, if you see a little light-blue box with a white frame in lieu of cover art in a Sonos controller, the track either has no art embedded in it, or the art is just too large.