Need a different way to put in a site-wide background

Site URL: http://lensesthatliberate.com/

Hi everyone, I had been using custom code for a few years now on my Bedford template site that allowed me to put a background image on the main of body of each page sitewide. 

Here is the code I have been using: 

The past week my site has been saying its insecure on many browsers, when I turn on the SSL certificate, this part of the custom code no longer works and no custom background appears on most browsers (including Chrome). Can someone suggest a different way of acheving the sitewide background that is complient with the SSL certificate? 

Thank you! 

Download the image and host it yourself on your site in custom files with URL you can add to your code. 

https://katescott.co/blog/how-to-upload-files-to-squarespace#:~:text=To do that%2C go to,onto the arrow to upload.