Using Responsive Lightbox by dFactory

Hello,

I have installed the plugin "Responsive Lightbox by dFactory", because I want to use it for a few images on the following page:

http://hilfe.macbay.de/neu/step-by-step-anleitung/

But I don't want to use it with all images on that page.

My knowledge about coding is a bit poor, so I wonder how to apply the lightbox-effect to single images. I tried to add

rel="lightbox"

within the <img>-tag, but after updating the page, WP has removed it.

Please help me :wink:

Thank you,
Sascha