So this choice depends largely on the Javascript framework used in the core of "My Little Forum".

There is no JS framework in the MLF-software.

This can be added by a simple <SCRIPT type=text/javascript src=..../> in the HEAD, leaving the choice of the lightbox/javascript-framework combination to the board's administrator.

The problem is the necessary PHP-coding providing the FORM for entering more than just the SRC of the full size image, and the creation of the appropriate HTML markup which can be picked up by the Lightbox Javascript package.

I believe that, if there are differences between the different Lighbox implementations at all, those differences are minimal, and can be handled by an MLF configuration option.