How to get the user photo on https?

Hello! I am trying to use https fully on the blog we have, but unfortunately the user photo is the only resource which is braking that and the green lock is not on because of it. We were looking for some time, but we were not able to understand where is the url created and how can we use there the //site.com/image.jpg format, in order to able to serve the proper version when on http or https.
Can you please help us? Thank you very much!

P.S. We were able to temporary fix the issue using the SSL Insecure Content plugin, but it’s adding extra unnecessarily weight on the server, so a direct fix would be way better.

Best regards!