Tap the round icon at the top of the navigation drawer to open the server info view.
Scroll down to the album art cache section.

The cache keeps downloaded album art images on the device. If album art was missing or failed to download MAFA will try again only after delay of few days.

If album art images were added later or changed, refresh the cache to attempt to download again.

Delete all cached album art images
Mark all cached album art images to be downloaded again