Is there a flag for downloadable pictures for only those who purchase an image?
I don't understand how the download feature works.
Does the shopping cart trigger the download, or are there other options that need to be configured so the customer can download only the items they have paid for?
In short, there's no server-side processing in a jAlbum album. It doesn't know anything about what a customer may, or may not, have purchased. It can't decide whether to allow a file to be downloaded or not - if there's a download link on the page, absolutely anyone can download the file.
The PayPal transaction is, in effect, invisible to the album - the album just passes the customer along to PayPal. That's the end of the album's involvement in the process.