When uploading images to post I get Parts of this page are not secure (such as images) warning on my URL

Site URL: https://www.interiorredoux.com/

When I apply images to my page, my page becomes insecure. The padlock next to the URL gets a yellow triangle on it and when I click on it, it says 'Parts of this page are not secure (such as images)'. 

A few months ago, I had applied the custom code advised on this page:  https://kerstinmartin.com/blog/site-not-secure  

and that seemed to solve the problem for a while but now it's back.

Thanks in advance for any help you can offer!