Have you checked the folder the tracks are held in for any album art image files and also check the track metadata tags to see if any album art is embedded in the track - software such as Mp3Tag will allow you to view/change the album art.
Also perhaps see this helpful link:
https://support.sonos.com/s/article/543
After changing the album art, don’t forget to also update your Sonos library index in “Settings/System/Manage Library"
Sonos grabs data from the tags for each song from your drive. It does not access an external database. Unless the tags on the file are incorrect, then I’d suspect some sort of interruption in the connection between your speakers and the drive, at the moment it is asking for an update for the image. Mine do tend to lag, by a second or two, but not much more than that, when I’m paying attention, which is pretty rarely, since I tend to listen to my music, not watch it.