Maybe just edit your config file ( config.production.json) to move your website name to https (or run ghost config url)? This should solve the mixed content issue.
This is definitely a misconfiguration issue. Fixing the assets that are output will only fix one symptom, not the root cause, and there may be other symptoms
As @hannah said, you have to reconfigure your URL. Login to your backend via SSH, type ghost config url https://yourdomain.com and restart Ghost. Make sure to re-upload the images afterwards, so Ghost replaces the URL for the image, too.