This is one of the (many) things that reading the manual could have answered for you...
Basically, you have two options:
1: - Embed better art (ask Google how to do this)
2: - Remove the embedded art and add file based art (again, ask Google how to do this)
As you can see from
this section of our fine manual, you absolutely cannot "override" embedded album art, there is a preferential order in which album art is loaded and embedded album art trumps everything else if it is present. There are many file based album art options, and many placement options for said files, but if embedded album art is present they will never, ever be used.
[Saint]
EDIT:
I don't think there is a lot of easy to use software on the Mac for removing or changing embedded images
You might want to look at
MusicBrainz Picard which can automate metadata tagging, and both embedded and file based album art (and, yes, even on a Macintosh
).