You want to download a photo you have purchased. You have two possibilities to do so.
Option 1: you can download a photo immediately after having paid your order either by clicking on the "Download all" button that will generate and start the download of a .zip file with all the photos purchased with this order, or you can download each photo purchased one by one by clicking on "Download" button next to the photo you want to download.
Option 2: Alternatively, you can re-download photos for up to 30 days after the date of purchase in your Download History. You will see a list of photos you've purchased in the past with your account, starting with the most recent photos purchased. You can click on "download" button to start eh download of the photo. If you see "Expired", it means you have purchased the photo more than 30 days before and you will have to purchase the photo again to download it.