I'm up to speed with importing and exporting cover art from a file. I can't find a way to delete or over-write cover art in a tag, though. Importing new cover to a tag with an existing cover seems to append the new cover without deleting the old one, and iTunes (for example) still displays the old cover. Am I missing something?
You can click on the 
icon in the tags dialog next to the cover.
Got it - thanks!