Remember the album art pane that was at the bottom-left of the iTunes window, in previous versions of iTunes? Well, iTunes 11 removed that. I’ve heard from a lot of people who used that pane to add album art to their tracks, dragging art there to add it to a track that’s playing.
You can still add art from the Info window: select one or more tracks, press Command-I, then paste or drag the artwork to the Artwork well, for multiple tracks, or the Artwork tab, for single tracks.
But there’s another way you can add album artwork. If you drag a graphic to the iTunes LCD – that’s the display at the top of the window, showing which track is playing – iTunes will add the graphic to the currently playing track.
It won’t add art to multiple tracks, if you’ve selected them, and it won’t let you add multiple graphics, but if you want to change artwork for a single song, while it’s playing, it’s a quick way to do so.