Hi,

This is a Featured Slider section of this theme, all the images you've seen are the featured image of the post.

You can change the Featured Slider settings on Appearance → Customize → Featured Slider, and change the image by replacing/editing the Featured Image of each post.

Read this article if you don't know about the featured image: https://www.wpbeginner.com/glossary/featured-image/

Regards,
PenciDesign.