I should clarify "then right click the artwork "box" (which will be empty)"
It is "empty" because you have selected multiple tracks from multiple albums, the reality is that the Artwork is actually preserved for each individual track and/or album.
So by "right clicking" the "empty" artwork box you will be offered batch processing options for all selected tracks.
The FAQ's of "MP3 Tag" are very intuitive too
Edit: "MP3 Tag" when re-sizing will attempt to use 500 x 500, if the artwork is smaller than that it will not "grow it" to 500 x 500
It's probably doing C but more effectively. A. does for me on mib2 high. Keeping it stupid simple. I manually pick out the best looking art work. I can see if you have a big mp3 collection you might want to wack them all in one swoop. Main thing is to ensure graphics don't pop the 500x500 limit.