jPages has a callback parameter which accepts a function. This function is called right after every page turn including when the plugin is initiated.
In this function you have access to two objects containg all the information available about the items and the links. Read more about this objects here.
This callback function is also the best way to add a legend to your pagination. Check how to do it in the following example.
You can also use this callback function to 'lazyload' images in your items. Read more about it here.