I have 2 cover art issues...
One stems from when I download music from a certain source. When I import the music, it uses the same cover art for all of the files (the cover for my album, Cowboy Junkies "Whites Off Earth Now") It seems to be a completely arbitrary/random cover art selection. If I select the files and then "Get from internet..." it does find the correct cover art, but I find this to be very strange behavior. And sometimes, if at quick glance see there is already cover art, I will skip past it and only catch it later when I'm playing the music and the Cowboy Junkies album appears.
The 2nd cover art issue comes from TV series. I have about 20 shows represented and MC17 finds the cover art for these shows. However, when I assign them and give it the okay, I still just see screencaps from the particular files making theater view browsing fairly poor.