I like what you did here. It's common for new developers to rely heavily on complicated Lightbox scripts, because it seems daunting to implement one on their own. Your script would be good for a beginner to review. It covers the 80% case. You don't say anything about mobile support or browser support, a note on that might be helpful to add.

I've got to say though, the underscores (while more readable) are not the "house style" for JavaScript (at least, not the native functions of the language), where everyThingIsCamelCase.