Why does my background image move on refresh/reload?

I recently changed my theme on my WordPress site and I’ve noticed that, since the change, my background image moves from the right to the left whenever I reload or change the page.
The entire page shifts to the right a few pixels if the page doesn’t have a scrollbar as well, and I was wondering what causes this and how to fix this, as it did not happen with my old theme.

My website is http://imperativeguild.com.