Want to add a click through popup dialog box when clicking on High Resolution button

As the title states I would like to have a popup dialog box appear when a user clicks on the button that downloads the full sized image. Basically it will display a custom text that states "this image copywrite blah blah blah. Click okay to continue." and then when they click okay the full sized images opens like usual.

How can I set this up in Gallery 2.X? Thank you.