I haven't found any .png artwork files yet. But if you open a .itc file in a good editor, and near the top (the exact position varies), it says "data" then 6 more characters and "JFIF", it's a modified .jpg file.
Copy the .itc file somewhere else. Then open the copy in your editor, and carefully delete everything from the beginning, up to and including "data", but nothing else. (Leave the 6 characters before "JFIF" alone.)
Save the file, and give it a .jpg extension. There is your art.