iTunes 12 has changed the Info window; the one you see when you select one or more tracks and press Command-I to edit metadata.
This window is especially different from the previous Info window when you select multiple items as it has an Artwork tab. You can add album art to multiple tracks by clicking that tab, then dragging the artwork onto the window.
But there’s a quicker, easier way to do this. Just add the album art to the square at the top-left of the window, where the musical notes are. When you do this, the album art will display, and the background of the top section of the window will change to reflect the colors of the album art.
Click OK to save the album art. Note that if you redisplay the Info window for the same tracks again, it will look as there there is no album art; iTunes, for some reason, doesn’t display album art for multiple selections. (This was the case in iTunes 11, but since the window was different – there was just an album art well – it may have made sense.) So, after you add album art to some tracks, and see the second window, if you view the same tracks again in the Info window, you’ll see the first window above.
You can use the same technique to add album art to individual tracks; the difference is that iTunes does display the artwork in this case, both at the top of the window and on the Artwork tab.