new theme will not resize images

I just came back to wordpress after leaving it alone for a year so I am a bit rusty. I do have a lot of hobbyist level coding experience though so it comes back to me quickly.

Anyway, on a new theme I am using, the images are not resizing to fit within the main content container, and it is leading to some images displaying whackily.

An exmaple of what I am talking about is here: https://www.radbrains.com/charting/how-to-use-fibonacci-retracements/

Is there any simple solution to getting my theme or wordpress to resize that for me?