You set the images to for download on the price list, this how you specify with images are available for download. Then the user will chose the downloadable size, if the cart detects that the size is downloadable the user will see the download link on the receipt page and if you are sending a receipt by email they will receive the links also.

You can sent the email receipt for user in settings > checkout > email receipt

make sure that you setup downloadable sizes in your price lists, the plugin assumes that you are setting up shippable products, if you want to make them downloadable check the checkbox next to the size name.