How do I fit images as backgrounds of each post

Yuuta theme but generally also?

Is there some easy way of doing it or do I have to go through the theme .php code and alter the div to have them fitted. Cause if that is the case it will be a liiiitle bit difficult for me at this point.

Also resizing images to fit each post manually by checking the dimensions of particular post is kind of….annoying to do.

Any insights?