After you have purchased an image you get a download link. You click on the link and the image will be saved to your computer. There is also an option to download with a zip file, which is handy if you have many images in the same purchase order.
After your purchase you can go to your home page and press Downloads in order to view all your previously bought images and download them again, as many times as you want. From this page you can also access all your receipts.
Contact firstname.lastname@example.org if you have any more questions or need any help.