Official Answer
Like all standard NationBuilder themes, Livingston allows you to upload your organization’s logo directly through the NationBuilder control panel — without ever editing any code.
To add your site’s logo, navigate in your NationBuilder control panel to Website > Site settings > Logo.

There, upload your logo file to have it take effect throughout your site.
Livingston will automatically constrain your logo on your site to not more than 40 pixels tall. We recommend minimizing your file size before uploading to improve page load times.