I know I need to use JJ2+ for that and a bit of angel script. However how would I do it? What I want to do is to replace graphics of certain enemies (functionally they would be the same, just with different sprites) with my own, but I don't know what are format specifications, would I need to make custom .j2a and bundle it and so on.

Here's the thread for that. I'll agree it's a bit confusing it's in the general jazz forum instead of the jcs & scripting forum, though in my defense it wasn't named that when I posted the thread.

Can't it be moved here? I mean, you're mod of this forum.

I am not, in fact, a mod of this forum.