Hello Richard E,
The certificate been used at "is5.mzstatic.com" doesn't match the domain name unfortunately (as it is issued to a248.e.akamai.net), this means that even if the images were to be requested via SSL/Https, you will receive a security warning from your web browser anyways, and until that changes, there isn't much we can do.
If you don't want mixed-content warnings been prompted by cover images, you will need to disable all external sources under Settings > Albums, and then purge the media library.
Regards.