Navigating around ul li and looking for the active state on the current slide
It looks like you're looking for the index of the first child of $imageContainer with the active class. (Incidentally, your code will also trigger on stuff like inactive, which might be a false positive. Be careful about your classes!)

Native selectors work fine here. Furthermore, your initial assignment doesn't do things the jQuery way -- there's no reason to initialise two separate jQuery objects. Try replacing your block with something like:

$imageContainer = $(this).parent().children().first().children()
currentIdx = $imageContainer.index($imageContainer.filter(".active"))


Better yet, instead of that monstrous .parent().children().first().children(), give it a meaningful ID or class, so you can just do $imageContainer = $(".image-container", this).

If you can guarantee that the ul will contain only lis, then you can simplify further to currentIdx = $imageContainer.filter(".active").index().

Depending on your exact situation, you may even be able to replace everything with the one liner:

currentIdx = $(this).parent().find(".active").index()