The feature you're asking for is the exactly same feature I've been asking for ages, and can be resumed as "the ability to represent a category field with a custom image", in your case you want to use a cover art for the album, and a different one for each song, in case of multiple artist album, in my case I had over 2500 albums from over 700 artists, and I can't figure out which artist is selected just by looking at a random cover art from that artist, and I am forced to get close to the computer screen to read the artist name what makes my remote control pretty useless, same thing applies to my video library, I had lots tv series box and animes, and I can't identify which series or anime is selected just by looking at a random screenshot taken from one chapter inside that field, and i also would love to represent each series with its own logo, each season with its own dvd cover art and each chapter with their own screenshot, and to represent each band in my music library with the band's logo, so I can watch my videos without get my lazy @ss out of the couch and listen to my musics without stop whatever I'm doing to get close to computer screen to read the band's name, and then my remote control can become really useful, instead of this useless brick that it is right now.
When I've asked this for the first time Rick.ca gave a workaround, but it actually doesn't work, the workaround consists in create a .log file, import it in to Media center tag it as a music file, then tag it under the artist you want then under the album you want, select a cover art for it, put the mouse over the album that your .log file is in and wait to the images roll, and when it shows the image you want just take your mouse out of it, but surely I don't had the time to put the mouse over 700+ artists 27 series 74 animes and 171 seasons to wait the images roll and stop in the one i want, plus every time you accidentally pass the mouse over a field to reach another one that its close to it, it will roll the images again and you need to redo this thing again, and when I close Media center and re open it, lots of cover arts (30 % or more) get reshuffled, it would be awesome to use your own custom image to represent a field instead of roll through the cover arts inside it, but actually you can't, and this have been asked for ages, and unfortunately probably you will hear a "the program can't do this, use a workaround that take ages to be done, doesn't work, and don't do what you want, or piss off" kind of response when you ask for this feature to be added on JRiver Media Center.