DISCONTINUATION NOTICE: Development on this plugin has ceased in favor of the carousel module in the Jetpack plugin. I recommend you use that plugin instead — it’s much more full featured.
Makes the native WordPress galleries use a lightbox script called ColorBox to display the fullsize images right there in the page. No modifications required.
You can also make single non-[gallery] images use the lightbox by giving their links a class of “lightbox”, like so:
<a href="..." class="lightbox"><img src="..." alt="..." /></a>
This plugin is available for download for your site.