I was hoping the protocol relative urls would fix this issue, but it doesn't. The SSL certificate used by Pinterest is not valid for the domain it is serving the images from (sorry for the dutch error):
media-cache-ak0.pinimg.com gebruikt een ongeldig beveiligingscertificaat.
Het certificaat is alleen geldig voor de volgende namen: *.akamaihd.net , *.akamaihd-staging.net , a248.e.akamai.net (Foutcode: ssl_error_bad_cert_domain)
As far as I know, the only way to fix this properly is if Pinterest would fix the SSL certificate. Another (workaround) would be to use the http:// version also from SSL, however this will trigger a warning in most browsers. Yet another solution would be to remove the Pinterest widget completely for SSL pages to prevent the error from showing.
Hope this helps!