To add a new slider item copy/paste a slide item and set image source.
<div class="owl-carousel main-image-slide"> <!-- Slider item --> <div class="item"> <div class="container"> <div class="row"> <div class="col-md-12"> <div class="slide-parent l-after l-before"> <div class="slide-parent2 l-after l-before"> <img src="image source" alt=""> </div> </div> </div> </div> </div> </div> </div>