Why is album artwork pixelated in only certain iTunes views?

I have a Mid 2014 MacBook Pro with all of the latest updates, including, in particular, iTunes 12.2.0.145.

All of my music is taken from MP3 files online; none of it is from the iTunes Store. Therefore, I provide my own album artwork. This is what the artwork for one of my albums looks like in the Albums view:

This is what it looks like in the Artwork tab of the Info window:

Just like in the Albums view, the artwork is displayed with sufficiently high resolution that it looks great with a Retina Display.

However, when I select the album in the Albums view (so that I can view the songs), it looks like this:

As you can tell from the Retina Display-friendly text on the left, this image is highly pixelated and is glaringly noticeable.

This occurs with every album in my library. Why, and (more importantly) how can I fix it?