This is my writeup on my fantastic experience with a wonderful piece of OS X software called Yate and it's developer Barry.
For many months I've had the problem of a poorly-kept music collection. Various files were missing tags, and since I'm using an Android phone with Neutron media player, half my music did not have album art on (Neutron is really picky when it comes to album art, in comparison to Poweramp etc.). As I'm using a computer with OS X, the software selection that could help me with this problem is fairly limited, so I wrote a post on head-fi and got a reply from fellow member TPGsanti that recommended I try out Yate. I tried the app out and was happily immersed with its capabilities, however by itself it could not help fetch many of the album art files. That's when it happened; I contacted 2manyrobots (the developer) and came in contact with Barry, a truly charismatic software engineer who took it on himself to figure out a way to get Yate to do what I wanted it to do. He thought that, since iTunes is really great at fetching album covers, he could find a way to get flac files into it (I still don't know why Apple doesn't support this format), and he did.
Without further ado, I present to you a quick guide on how to fetch proper album art for your music files (be it flac, mp3, mp4 etc.). You can find a lot of this, as well more, in Yate's help file.
Firstly you download Yate. You can find it here: http://2manyrobots.com/yate/. Afterwards, you can use it's own "Auto Search" function for album artwork, or you can use it's batch processing and send your FLAC collection to iTunes where even better album art can be found.
You start a new action and select the function called "Create Stub"; this will create a small mp3 file in each folder that Yate detects there is a music album without album art (it will ignore folders that already contain artwork). Even a single FLAC file is enough for Yate to go look for album art for it, which for my collection is brilliant, as I tend to pile a lot of different music in one folder. After you run the action, Yate will create a stub file (a dummy file) for each album that requires art, and automatically add those files to iTunes.
Aftewards you go to iTunes where you will find a bunch of new entries in your iTunes library. These files will have a grouping field named STUB, for easier access. Make a playlist with those files, select them all and from the context menu select "Get Album Artwork", which will make iTunes locate the missing album art. After this is done, you go back to Yate and make a new action in the batch run context menu, using the "Process Stub" function this time. This will extract the artwork from iTunes and insert it in the original music tracks, and then it will automatically remove the stub files from the iTunes library and the hard drive.