Sell Photo plugin allows you to create a Buy button for each image of your WordPress photo gallery. When a user clicks on the Buy Now button, they will be taken to the payment page where they can pay instantly using a credit card or their PayPal account.
When you insert a photo gallery the shortcode will look like the following:
[gallery link="file" ids="687,688,689,690,691,692"]
All you need to do is add a “sell_photo” parameter so the plugin can create a Buy button for each photo of that particular gallery.
[gallery link="file" ids="687,688,689,690,691,692" sell_photo="1"]
For documentation please visit the WordPress Sell Photo plugin page.