Good point. Tracked here:
http://www.ventismedia.com/mantis/view.php?id=11869

-Rusty

Thanks for the quick answer. I also thought that MM considers it as a new file. But why not just rewrite the tag with Album Art? If I modify any other tag MM only updates the Metadata. So there is a lot less traffic.

That's because modifying the artwork in the tag actually modifies the file, so it's considered to be a more recent copy.

-Rusty

After I add album art in MMW for tracks that already synced with MMA it downloads the whole track again instead of just update the Tags in the next sync.
"Sync all Artwork to file folders" and "Copy Artwork to file tags" are both unticked in options.
MMW 1691
MMA 224

PS: it only downloads tracks again, when art is stored in tags. When art is stored as jpg in folder it only updates the tags during next sync!