I use as the target Lightbox-software Slimbox (www.digitalia.be/software).

And there it begins. Wich *box should be supported, wich not?

This is to the developer(s) of MLF to decide.

Since those Lightbox packages rely on a given DOM inspector of a Javascript framework which runs triggered by the evend OnLoad of the HTML document, and creates the event handlers which the lightbox package then uses.

So there are two different versions of Slimbox, one for Mootools, the other for jQuery.

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

I would just recommend to chose one implementation which can display a caption/description to each individual image when shown in the slide show. Without that, the slide show would not be accepted, accompanied by complaints like "I do not want to constantly switch between slide show and the text for the image".