As I mentioned earlier, my reason for even bothering about this was my frustration with missing artwork on my mobile devices. I actually contacted Apple, but they weren't any help. As far as they're concerned, missing artwork is non-existent with iTunes-only purchases; the problem is with CD-ripped music and those from other sources such as Amazon, BeatPort, BandCamp, etc - i.e. the competition.
Well, after trying various methods I'm now getting some success. The key it seems is to make sure that every file has the artwork displayed in the bottom 'preview' area when you "get-info". This of course is outside of iTunes and Yate. Even if the artwork appears fine in iTunes or Yate, it won't necessarily be present when you go 'get-info'.
Strangely enough, the only way to have the artwork appear in the bottom preview area and thus on the mobile devices is to use both iTunes and Yate. I have yet to see the rhyme or reason to it, but it works.
I usually start with iTunes. Selecting ALL the tracks on each album and then 'get-info' will bring up the info window. Next make sure that artwork is checked then I copy & Paste the highest resolution artwork I can find in the little box. Click OK and let it do the embedding. If this doesn't fix it, I'll use Yate to do the same. If it's still not doing it, I go back to iTunes and repeat. In most cases, the problem is fixed after using Yate. Sometimes, I've had to use iTunes 3-4 times before it would take.