Spent some time this morning with the latest BETA build:
S2.25 2016-04-05
Build: Manic Moose
MPD: 0.19.12 NEWS
Kernel: 3.16-0.bpo.2-486
... thanks for adding the Soundirok covertart.php script into the build - very helpful.
I tried this out, after renaming all of my 'artists' images to 'thumb.jpg' to match the script's defaults and all of my artist images loaded, just fine. I also looked at my albums and with the exception of roughly seven (7 out 1200+), all of those loaded, as well. Not sure what was wrong with a handful of albums. It was easy to locate the missing album covers in the app, so I re-embedded the artwork and ensured that each had a 'folder.jpg' (they already did), but no dice. So, I went back to using the 'Collection URL' and 'Cover/Artist Thumb Filename' settings in the app and everything displays nicely.
While I was at it, I looked (again) at some of the many album thumbnails not displaying in MM and have concluded that MM simply is ignoring the 'folder.jpg' file, if it has problems processing the embedded artwork (or there is no embedded art work). I even tried re-embedding and/or scaling artwork for some of these to see if that changed anything; it didn't, even after clearing my browser cache.