Full screen zoomer is a simple plugin that make your image full screen when cliking at it.
Simple to use :
1- install the plugin
2- nothing to configure, just add the css class "fullscreenzoomer" to your image
3- That's it ! enjoy !
Just any plugin in wordpress, no custom things !
To learn how to install a wordpress plugin, please visit this step-by-step tutorial :
http://www.wpbeginner.com/beginners-guide/step-by-step-guide-to-install-a-wordpress-plugin-for-beginners/