Add product thumbnail to user/%/purchased-files?
You need to change page callback for path user/%/purchased-files in hook_menu_alter() and your custom callback should be based on code of uc_file_user_downloads() function, you need to change it, add new column with product image. I guess you need to change query in that function to get more data from 'uc_file_products' table about the product id, and based on that you can get all product info etc