A normal & working link of your plugin will be:
<a href="http://blog.melroy.org/wp-content/uploads/2014/01/ubuntu13.10-unity-menu.png" rel="lightbox">
However, when I choose the Gallery feature in WordPress to add pictures to my post AND I select 'Link to': "Media file" instead of "Attachment". I get the following links in my gallery:
<a rel='attachment' href='http://blog.melroy.org/wp-content/uploads/2014/01/Linux_mint_menu.jpg' rel="lightbox">
You see a double rel in the a tag. This link won't work :(
You should support the native WordPress Gallery feature! I hope you can help me.