It would seem that Chrome is the only browser currently flagging these images as unsecure. If we can get any more information pertaining to the 'why' or 'how' to workaround the flag we will update the community here.
Our customers are complaining that the webinar registration pages presented and hosted by GoToMeeting are being reported as InSecure in today's modern browsers. This is because the customisable logos we upload are hosted on a service, provided by GoToMeeting, that is only presented over HTTP (http://images.gotowebinar.com/...) rather than HTTPS (https://images.gotowebinar.com/...). As the registration page is presented over HTTPS, this conflict is causing our end users concern and reducing registration take-up. Surely GoToMeeting could add an HTTPS capability (Let's Encrypt SSL Certificates are even free these days) to secure the images and prevent browser warnings and customer concerns?