Thanks for linking to that article, but as mentioned previously, while the method described should work, there's a bug that prevents the correct artwork from being displayed on some Macs, despite all artwork files being present and properly named — and the methods described in the article unfortunately do not compensate for that bug.
I confirmed this previously by observing that my custom art for genres and the correct, stock images (as the case may be) are in fact being displayed on my secondary Macs, but not on my primary Mac (the one on which I actually assigned the artwork). In other words, I'm setting the genre art on Mac 1, and iTunes match is correctly sending those files to Macs 2 and 3, causing them to display the desired artwork. Yet, on Mac 1, that custom artwork still refuses to display. And in the case of stock images, all are present and are correctly named on all 3 machines. Yet, only Machine 1 refuses to display it. (At the time, the machines were running a combination of Yosemite and Mavericks. But now, they're all running Yosemite.)
So, on affected machines, something else is preventing the display of the artwork, despite all files being in the correct place.
We have yet to identify the source of this bug or its true fix, and it's now been extant for years. Until someone identifies the source of this bug, all we can do is continue to report this to Apple, in hopes that enough reports will cause them to take a look.
Even if you've done so in the past, I urge everyone to take a moment report it again:
iTunes feedback:
http://www.apple.com/feedback/itunesapp.html
Thanks.