Bingo - it is not the size, but your question did lead me to find the actual cause.
I do not know how to change the size of an embedded art - I guess it is probably no different than any other picture, but I do not even have other sizes of the existing art to try. I purchased all these files as .mp3 from ZBS, and they all came with the embedded artwork.
At first I thought you might be on to something with that idea, as I used the Artwork tab of the Edit screen to look at the file sizes (shown briefly when hovered over). I noticed that almost all of the images that WERE being displayed in DLNA were smaller than 157KB, while the ones NOT working were all larger. And then I found one that was being shown just fine at 448KB.
But then I FINALLY spotted the real issue - when I hovered over the ones that were NOT working, it showed [image/png], and all the good ones were [image/jpeg]. I had not seen that information before since there are no file names to look at! Now all I have to do is figure out how to extract those embedded images and change the format to .jpg.