first thing you might want to consider is simply embedding the album cover in the MP3;
that's how we do all of our tracks, and if you purchase an MP3 from Amazon or other download retailer, the album cover is usually embedded as part of the meta/id3 tags. Not only will that solve the issue of artwork, but it's only fair to properly embed all of the necessary meta into the files you are selling to make it less work for your buyers.
as far as the artwork field, i always thought of that as a vestige of the old maian media script, perhaps implemented before people were embedding artwork into the audio tracks; But in any case we always use that field for the PDF booklet download, since the albums we sell are all issued on Compact Disc, we want the buyers of the download to also be able to read the PDF version of the CD booklet.
So once you embed your album covers, you can just ignore the artwork field, and this won't affect anything, since the field will be null, there won't be a download button for that.