I'm not sure when this started happening, but I noticed today in Google Chrome (and subsequently in Firefox & IE) that my Joomla 3.9.20 site is causing a "Mixed content" warning. I searched the .html source for instances of http://, and there are none.
Using Google Chrome's developer tools network tab with the filter mixed-content:displayed, I see two requests, in the name column it says www.my-site.com (well, it says my actual site's name), in the status column it says 302, in type it says "text/html", in the Initiator column it shows the path to two .png images (existing_image_thumb.png) (i.e., it seems to be trying to get _thumb version of two existing images). I don't understand this. How can a .png image initiate a request for the main page?
I did notice that some other images displayed on the home page that are not causing this error have a _thumb file in the same folder as the 'normal' image, but the two images causing the issue don't have that _thumb file. Could this be the issue? How are those _thumb files generated anyway? The site uses a RocketTheme template, perhaps the _thumb file comes from the template?
How do I diagnose, and fix, these mixed content warnings?