With this method, iTunes’ll go through your entire library looking for missing album covers; if you only need a couple of items done, you could also right- or Control-click on an album and choose Get Album Artwork instead to save yourself some time.
In either case, you’ll be required to sign in if you’re not already.
That’s great and all, but what do you do with the albums that iTunes can’t find artwork for? It’s safe to say that this ain’t pretty:
And neither is this, which is what iTunes thought this album cover should look like. Eeek!
At least I get to make a Hulk joke here, right? KARAOKE SMASH! Right?! Guys? …Sigh.
Anyway, let’s go find some proper art, shall we? What you'll need to do is search the Web for the album, and then once you've found the correct cover image, right- or Control-click on the picture and pick Copy Image from the contextual menu.
Then open iTunes and find the album that you’d like to add the image to. Right- or Control-click on the album and choose Get Info…
…and click “Yes” on the dialog box to confirm that you want to edit multiple items.
Afterward, you’ll make sure you’re on the “Info” tab (or on the “Artwork” tab if you're only adding the cover image for one song), and you should see a box in the lower-right corner labeled “Artwork.”
Select that box to highlight it:
Then press Command-V to paste in your copied artwork.
When that’s done, click OK to confirm your changes, and all should be right with the world. Well, with the world of that particular album’s art, anyway. Doesn't this look much, much better?
Inasmuch as one can really call those hairstyles “better,” I guess. Hey, don't judge me for my musical tastes, or I'll force you to listen to me singing “Muskrat Love.” Then you'll be really sorry.