Default sidebar to bottom of screen

Just started using the default theme, I like the simplicity, would like to make it more simple. When I put 600 pix wide shots on the page, it shifts the sidebar to the bottom of the screen, which I like, get it out of the way for a clean photo presentation. Can someone tell me how to make that change permanent?
Also would like to have the photographs center on the page, and the side bar seems to throw them to the left.
Is this an easy fix?
https://www.oboylephoto.com/blog