HTTPS mixed content warnings

This issue can commonly occur when user sets SSL certificate on a website where it wasn't present and installation wasn't configured initially in that way. You could try using some SSL links fixer plugins like maybe following ones https://wordpress.org/plugins/wordpress-https/ http://ithemes.com/codex/downloads/fix-ssl-non-ssl-links.zip but that doesn't guarantee fixing the issue. More proper way would be to remove images that are causing this problem and re-uploading them again. By using google chrome developer tools console you can find list of images that are causing this issue.