Losing Artwork seems to be a reoccurring issue due to changes that Apple makes to either the IOS or iTunes. The only way I have found that work properly, without losing a lot of space in the 'Other' category (happens with 'Send To' feature), is to reset the phone and restore it with the latest iTunes backup, then sync with MMW.

It would be very handy if we had a flag that we could check to force the sync function to update all files on the device, even if there was no file change.