Change which Matrix items displayed
In your Twig example your '.picutures' class name includes a leading dot. Maybe not what you want there.

Also in your slick configuration 'slidesToShow' should be 1 if you only want to show one slide at at time. 

And slick is going to operate on the direct descendants of the selector class identified in the configuration so the intervening divs may be causing a problem depending on the associated CSS.