This plugin provides a very lightweight, minimalist, jQuery-based lightbox for displaying full-size versions of the images from galleries on a WordPress site. It also gives some basic copy protection that should prevent relatively unmotivated image thieves by inserting a transparent overlay over the lightbox and all gallery thumbnails. This will prevent people from copying images in your galleries via right-click or drag-and-drop.
It’s not a true preventative measure, in that someone who really wants to download copies of your images will most likely find a different way, but it is a little more secure than linking directly to the full-resolution versions. Image copying is pretty much impossible to fully prevent aside from just not publishing your work in the first place; this plugin’s goal is to strike a good middle ground.