Fix missing iTunes album art with blissiTunes does not show album art for all of your music. A reason for this is that iTunes sources album art exclusively from the iTunes store. If your music isn't featured in the iTunes store, iTunes won't find the art. This is common for rarer releases, older releases, cover mounted CDs or bootlegs. It also affects music publishers who have not signed a deal with Apple, whether small (independent musicicans) or not so small (The Beatles!).
If album art is missing in iTunes, you need to either insert it manually (here's how) or use software like bliss to find the art and insert it for you. Here's a walkthrough of the latter.
Right click on all the selected albums and choose 'Get Info'.
Don't click any of the check boxes - just click 'OK'.
And finally we have our album art!
It's important to note that bliss hasn't read the album art inside iTunes; bliss only reads the music files themselves, and the folders in which mp3s are stored. This means, when you "Get Info", the new art overwrites the art in iTunes.
In the future, I'd like to streamline some of this in bliss. For instance, the latter stages where all of the albums are selected and you choose 'Get Info' could be avoided if bliss instructed iTunes it has updated the music files and to re-fetch the album art from the music files. If you want to see this kind of functionality, make a suggestion on the UserVoice pages.
That is how to use bliss to fix missing album art in iTunes.