The images are not expanding, they are stacked one atop another inside a movieclip and are being moved about on mouseover commands. There may be a minorly elaborate scheme of deciding how pieces move where when, but I don't think a tutorial would have been written for it... though I could be wrong. The implementation has a couple of flaws in that sometimes when you intend a particular image to be revealed, the motion of things causes another image to move under the mouse, and if you move your mouse off the left edge, the thing reverts to showing the far left image.
Chances are you can design this using a bit of math logic and a coded tweens for moving things.
Instead of using lightbox in your search, just try things like sliding image gallery
I'm going to need a little more info on what you mean by stacked together in a movie clip. I took a stab at this and first I tried having all the images on their own layer which wouldn't work and then making them all buttons and placing them all on the same layer and have different frame labels show you the different views but I get a lot of error messages about the code for my buttons not working. I don't know is the problem is a button laid over another button so I tried adding a button layer of transparent buttons and that didn't work either. The template says Flash 8 which what I have and am trying to use. Also no luck on searching "sliding image gallery" they are all full-size images that are replaced with full-sized vs. this dynamic looking expanding and contracting menu.
I'd appreciate if you could tell me more specifically how you would build this movie clip. Thanks!