Show full size image on click

Hi,

I have changed the plugin slightly so that the images display in a lightbox when you click on them instead of a separate page. I was wondering is there a way to view the full size image when you click on it?

I thought maybe if the plugin didn’t resize the images on upload but then used CSS to make the images smaller, but I don’t know how to do it. Is there an easy way to do it?

https://wordpress.org/plugins/wp-responsive-thumbnail-slider/