This functionality is available in the Pro version.
Follow the next steps in order to add your custom button to the particular Gallery image lightbox:
- Open your WordPress admin dashboard.
- Navigate to the "Edit Gallery" page.
- Find the "Lightbox button URL" field and insert your PayPal URL: http://prntscr.com/8f1gw2 (for a version older than 1.5.1: http://prntscr.com/71cpdq)
- Click the "Update" button.
Button with the default "Purchase Information" title will be added to the lightbox.