EWSEL Lightbox For Galleries (ewsel-lightbox-for-galleries) free WordPress plugin

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="http://domian.com" class="lightbox"><img src="../img.jpg" alt="Demo" /></a>

developer version : Github