Image size help

I’m using the Daily theme from here:

https://www.robertbrodziak.com/en/free-wordpress-themes/daily-theme/

Right now all image are constrained to 640px. How can I post an image that’s larger than 640px? I’m trying to post images that are 1000px full width. When I tried to change the img css field, I can change the width of the image, but the image quality decreases dramatically.

This is my website:

www. theworkprogress.com