Website with blocked mixed content

I'm trying to fix an SSL error. Firefox alerts that the website isn't secure and displaying mixed content. I've visited their support page https://developer.mozilla.org/en-US/docs/Web/Security/Mixed_content/How_to_fix_website_with_mixed_content but my images are already showing https? The program LinkChecker displays this error Error: SSLError: hostname 'www.socreative.graphics' doesn't match 'socreative.graphics' What steps do I need to take to resolve?