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.
I'm a big fan of doing things easily. Maybe Apple meant to make this change, or perhaps it was an oversight. It could be part of the big package of iTunes GUI changes that have troubled many users. The fact that you can still get to the feature by pressing the Option key indicates that the changes were by design. Anyway, the older and faster method is still around; you just have to know the "trick" ... and now you do.