add background image to site

my site is https://www.sportingjobs.org/

i want to add the logo at https://www.sportingjobs.org/wp-content/themes/jobroller/images/exlogo.png as a background image to the left of the page

something like they have done at https://www.wphired.com/

cant find which file to add it as so it appears on left like the ‘w’ does in the example above

thanks!