I haven't tested in Lightbox2, but it looks like in lightbox.js there is an option named this.fitImagesInViewport
. Try setting that value to false
and see if it gets the result you're looking for.
Edit: I ran a quick test, and it looks like that should do it. You can take a look at the source for lightbox.js here, so hopefully the change in your code should be fairly trivial.