According to its creators, "Lightbox is a simple, unobtrusive script used to overlay images on the current page. It's a snap to setup and works on all modern browsers."
And it really does what they say. So, why not use it with DNN? The fact that it can also work with groups of pictures providing basic back/forward navigation functionality makes it ideal as a lightweight picture gallery. Of course, it can be used with single pictures too, eliminating popup windows in order to see a picture, complete with (optional) caption, at a larger size.