You can add this meta tag in the head to block external images using http
HTML Code:
<meta http-equiv="Content-Security-Policy" content="block-all-mixed-content" />
I added that on my site. I also use CloudFlare to enable "Automatic HTTPS Rewrites" option which helps fix mixed content by changing "http" to "https" for all resources or links on your web site that can be served with HTTPS.