Add your markup for the download button to the template that lightbox uses. Lightbox hijacks all the clicks to the plugin, so you have to add an event handler to your new markup. Do this in the same start function that you added your markup in. You can use
window.open()
to open the link in a new tab.
Then you can populate the link based on
this.album[this.currentImageIndex].link
later in the updateDetails function.