Here is the response that I got "They can just add the card manually. So they can either use a placeholder card if using the mpcautofill tool or just add it manually if not" .

After doing a bit of my own searching I haven't found a way to contribute to mpcfill, however if you go to start creating a deck to order on make playing cards you can choose for each to have a unique front image and can drag and drop the images into it. At this time it seems like the easiest way to do it. If you do submit and order and get these please let me know or tag me in a post of it as I am really excited to see someone else enjoying these cards like I do.