Having issues with featured image

I am working on my following site: https://www.zomnomz.co and I am a complete n00b.

Regardless, my site is image-heavy. In order to use size scaled images to increase page speed, I have uploaded resized thumbnails (typically 192px wide) of my original images (typically 600px wide or less). So my media library has both the 192 and 600 available. I set the 192 as the featured image hoping that wordpress will put it on my home page. However, wordpress takes the 600px image and resizes it to 192 and puts that on my homepage. In fact, wordpress just takes the first image in the post and uses that as the featured image regardless of what I’ve designated as the featured image.

I’ve run this as many ways as I am capable of doing but I’m coming up short. Any help would be greatly appreciated. Thanks!