My site is loading images from other sites and this is causing warnings when I implemented HTTPS instead of plain HTTP. I know why this is happening but I'm wondering how to correct. 
Best solution I have seen is here, but I don't understand how that works.
The poster suggests prepending https://example.com/imageserver?url= to the image url. This doesn't work. So what am I missing? What is imageserver?
I hope this makes sense, I'm not sure if I'm not just missing something obvious here.
 
     
    