I’ve tried to resize my images to fit in the slider but since some of the photos are vertically taken and some are in different scale/proportion, only part of the pictures can be shown in the slider, basically distorted. Is it anything I can do to display images properly in the slider, like adding CSS?
You need to process your images before using them in the slider for them to have the correct sizes.
A square cannot fit in a rectangle and at the same time fill it.