Hello pentangle,
Does the album art appear correctly on the iMac you use to sync to your iPhone?
If it does, there might be an issue with just the Music app itself on your iPhone. Since we know it is still syncing the item, you can remove the app and reinstall it again which might refresh the app and the sync process to force the album art to sync with them: Remove apps from iPhone - Apple Support
If it doesn't, you would need to see about reimport the songs again as they might be having issues which can be resolved by reimporting them.
If you still have issues, the best thing to do is reach out to Apple directly as they will be able to have a deeper look into this concern: Get Support
Thanks and have a wonderful day.