Motti,
Shalom!

Lightbox works really well for me in unison with IH2.
I maintain only one size of images (LARGE), and keep them in the main /images folder. The folder that _LRG and _MED are in, but NOT _LRG or _MED.

Lightbox finds them just fine and they display at whatever size they happen to be. It really works great.

To have additional images for Photo.jpg for example:
Photo_01.jpg
Photo_02.jpg
etc...

Simply create the images, and move them into the /images folder.
Then when you define the product in admin>categories>products
click the browse button, point to the photo in the /image folder that you want to be the main image, and lightbox will do the rest.
I assume it does so based on the part of the name to the left of the underscore.

I can't imagine this being any clearer.