Small image lightbox popup issue

We have a template that has a "portfolio" with images and when you click on the image, you are redirected to a new page. We don't want that, we just want to make the image shown in a lightbox/popup. So we installed the "Image Formatr" plugin, but now, ALL the images are processed and shown in a lightbox. We want to have the ability to include, or exclude images from processing. Both options would work for us. By the way, we are not bounded to that plugin. Other idea's are welcome.