I think everyone may want to stop messing around with ID3 tags. It took a while but I started getting some album covers. That's when I noticed that some of them don't match my embedded images. Think about that for a sec...
Only thing that can mean is that Groove is getting its images by matching Artist, Albums, and Songs to a central database. ID3 tags or cover art images in PNG or JPG, it all doesn't matter. You just have to cross your fingers and hope that how you named those THOUSANDS!! of songs to Microsoft's liking and Groove makes the match in their DB. By the way, this is all on PC so far, I haven't tried my Lumia 650 yet. Fingers crossed.
As for PC users, my recommendation is MusicBee. Great program that is very actively in development. It has ALL the options you can think of and some you would never have thought of. MusicBee works just fine with either embedded images or image files in folders, take your pic. It even has a layout a little like the Groove PC app which I sort of liked.