I've found it hit and miss with displaying embedded images.
I took one of my favourite images, and saved a copy scaled at 90%, another at 80%, etc down to 50%, with 75% thrown in for good measure. I saved the 90% scaled image to the first song in the album folder, 80% in the second, and so on. This was all saved on my USB stick, and replaced into the car.
The 90% scaled image still didn't play, but the 80% one did. All the smaller images played as well.
So for this image, the limit was around 345 x 345 pixels, and 62 kB file size. I have had images larger in pixel size and in file size work before. The key seems to be to keep the images small.
So now I've embedded the 80% scaled image into all the songs in that folder, except for the last song, which is an extended version. I found an alternate image, and embedded a 345x345 px version to that song.
There is plenty of resolution on the infotainment screen, and the images increase the file size of the MP3 file by only a tiny amount, and storage is so cheap, that it's a shame I can't use higher resolution images.