iTunes 12.5.1 selected movie artwork appears pixelated

System: MacBook Pro 15-inch, Late 2008 (not retina), 2.4GHz Intel Core 2 Duo, 4GB 1067 MHz DDR3, 500GB HDD

Software: iTunes 12.5.1 on OSX 10.11.6

Problem: iTunes currently selected (zoomed in) Movie artwork appears pixelated, as if it were scaled-up from a lower resolution. This happens even when resizing the window (which shrinks the image). The artwork displays properly in the "Get Info" window. Music, Television, Audiobook, Podcast and iTunes U artwork seem unaffected – it only happens to artwork with a portrait aspect ratio (movie posters).

Troubleshooting tried:

• Deleted ~/Music/iTunes/Album Artwork/cache – helped only once until iTunes was restarted. Repeated attempts have failed.
• Pasted both square and portrait artwork onto an imported MP4 movie, confirming aspect ratio problem.

Anyone have the same problem, or an idea how to fix this?