Consider the situation that you are using the fancy box module in your drupal 7 site and you have to display the images when clicked on in a pop-up.You just need to follow the steps mentioned below which can guide you through the various procedures.
Install and enable the Libraries API ,the jQuery Update module and the fancyBox module.
Go to admin/config/development/jquery_update and select the jQuery version to 1.7 or above.
Download the fancyBox plugin and extract the directory into sites/all/libraries.
To configure your image fields to use fancyBox, go to admin/structure/types and click "manage display" link next to the content type of your choice.
Under the "format" column, select "fancyBox" next to the image field. You may also configure the options for this field.
Now when the image added using the content-type will be displayed with the fancybox pop-up when clicked on the image.