where do I change the default gravatar http to https

Hi,

New to wordpress and have it integrated into a magento store via the fishpig integration tool. The whole domain is secured via ssl. when a user comment on a post the default avatar or the generated gravatar throws up warnings “unsecure content” I have read that I need to change the default http://gravatar.com etc to https://secure.gravatar. But for the life of me I canot figure out where this is I have looked in lots of files but have had no luck finding the address to change.

Any pointers to where this file / address is located would be greatly appreciated.

Kindest Regards
Tony