I found a workaround for you. You'll need an external tool named Mp3tag besides MC. (Download the latest Mp3tag from here:
http://www.anytag.de/download/mp3tagv236hsetup.exe)
Here's a step by step:
1. Make a smartlist that uses No Duplicates for Album so that the list contains only one ogg file from each album.
2. Select all files in this list and convert them to MP3. Use the fastest and smallest option (i.e. custom command line:
gogo.exe -b 32). Configure it to convert the files to a new temporary folder and don't let it update the database.
3. You can mass extract the cover art files with Mp3tag. For this you need to define a new "action" in Mp3tag:
- top menu: Tools > Options > Actions
- make a new Action Group (press the star button)
- give the new action a name
- now in "Action" dialog window, press the star button
- select the action type: "Export cover to file"
- the next dialog window asks for the filename format string. Type this string:
%artist% - %album%- Enable also the "Show selection dialog" option. This allows you to select which actions are used when running the Action command.
Open the temporary mp3 folder in Mp3tag, select all files and run this action (top menu: Convert > Actions). It will extract the images in the same folder. If you have multiple artist albums you may want modify the MC smartlist rule and handle them separately. For multiple artists albums the correct filename format string in Mp3tag is
(Multiple Artists) - %album%EDIT
An album name like "Best of" is also a special case if you have many of them. Perhaps you can make a smartlist that takes this into account by using the "Filename (path)" field instead of the "Album" field.