And nowhere you mentioned SEO concerns? Do you understand that links (either textual or thru pictures) to websites or pictures can leave your website scope? The resource may not even support HTTPS, so it can't be your fault to not use HTTPS in the link - likewise it may only support HTTPS and no HTTP anymore (not even a redirect). And search engines should know/understand that better than most of the website owners.
A good question for the google's search facility, indeed.
That warning emanates from some of the latest versions of browsers and has nothing to do with SEO
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