So I was updating some of my albums with high-res album art from album-art.net and fanart.tv and noticed that the images didn't look too nice in the Music app. These images are generally 1000 x 1000 pixels. I took a screenshot on the phone and measured the dimensions of the album art as displayed on the screen. It's about 760x745, so I tried resizing my album art to 750 x 750 and then copying the MP3s back to the Z10. It makes quite a difference on the phone.
Here are before and after screenshots, although it's not as obvious just looking at the thumbnails: