How do you alter the album covers on the new iTunes? The software has now loaded a number of inaccurate album covers, and I want to fix them.

I just downloaded the new iTunes, and I'm already seeing one big problem--I can't seem to figure out how to change the album covers. A number of the images are inaccurate, but I can't seem to figure out how to take images from my desktop and get them into iTunes to replace the ones already there. I know, it will work well enough without the right pictures, but album cover art is something I like, sometimes almost as much as the music itself. Any help out there?