How to reset album artwork

[hannibal]

I have about 50K+ mp3 files in my media drive. And while Subsonic is working fine in terms of playing everything correctly, the artwork for MANY of the files is completely wrong. I have my library organized by <letter of artist name>/<artist>-<album>....so if I go to the O folder, there's a folder called 'Paul Oakenfold-Bunkka'. And within that folder are all the songs. However, under the root O folder, I might have a few one-off songs where I didn't want the whole album. There's no artwork files in the folder but yet when I look at some of the songs in the browser, it seems to be grabbing random image files and tying it to those songs. Furthermore, I don't seem to be able to manually change any of the artwork for any of the albums. I can go through the change process, but then when I go back to the list in the browser, none of the changes I made (through downloading the correct artwork) are there.

I'm running this on Windows 8.1 and I'm on version 5.2.1 (build 4428).

If anyone could shed some light on what I'm doing wrong and/or what I can do to either change the artwork files or just clear them out and start it over, that would be VERY appreciated. Thanks for your help.