Photo quality goes down when achanging photo size

When I upload a photo, the quality is good but weirdly when I make the width smaller fro 1200px to 850px, the photo quality goes down and it looks blurry…

Why is this happening and how do I fix it?

Thank you!

Here is my site Read-Only: [LINK][1]
([how to share your site Read-Only link][2])