I am very dedicated to getting the proper album art on the hundreds of CDs I've put into iTunes. Until version 12 arrived, it was a pretty simple process. You could Google the album art, then simply drag the album cover from the web page to the designated spot after selecting all the album tracks and doing a "Get Info" (Command-I) from the File menu of iTunes.
That all changed in iTunes 12, for reasons that escape me. Doing the same sequence now gets you a newly designed dialog box that wants you to load the graphic from a location -- no more drag and drop. It takes more steps to add the album art, and who knows why Apple made the change.
Don't despair! The old tried and true method is still there, but hidden. Just hold the Option key down before you select "Get Info" to bring back the old GUI that lets you drag and drop your album art right into iTunes. You can see both versions below: the current "improved" design and the "hidden" old version that you can restore by pressing the Option key.