you would need to modify the store to integrate the file upload process.
introduce a link in a relevant place to take you to an image upload section of your site , and use dfp2 or the image uploading element of universal email to handle the images.
you would also need to add tables to your database to store the image links and relevant data such as orderid's
once the upload is complete, you could then navigate the site visitor to the cart, or wherever is appropriate