No files needed, as only the metadata is necessary to do look-ups.
You're on a Mac, and I don't know which tools are useful for album art downloads (outside of iTunes and scripts for iTunes).
Beyond MC's Cover Art > Get From Internet..., you can use MC's Links feature. Select the album's tracks, and the click the
Images link in the upper right (to the right of the tabs). This will take you to Google Images, using the Artist as a search. But some of us find Artist too wide a search, and modify the link to include Album. For more info on modifying Links, or adding your own links to other artwork sites (such as AlbumArtExchange), see:
http://yabb.jriver.com/interact/index.php?topic=83371.0So long as you can find artwork on the net, you can paste its URL (or image) via MC's Cover Art > Paste From Clipboard (Image or URL).
I have not released the update to my discogs script that downloads cover art for a release. Since this artwork is available for viewing on the Discogs site, unless you are wanting to download all the cover art available, there may not be much benefit over just copying the URL (from your browser, Copy Image Link) and Pasting to MC (Cover Art > Paste From Clipboard (Image or URL). If you had a bunch of albums already populated with the Discogs Release ID, then you could grab artwork quickly for all such albums. But you should familiar yourself with the script and usage first, to see if it is something you care to use. If you want the image downloading, I'm happy to post an update.
Note: when you have the choice of copying the image itself, or a link to the image, copy and paste the link, because you'll get higher quality artwork (this avoids format conversion to/from the clipboard).