The error you’re getting indicates that there is at least one component of the page that is retrieved from the server using an unsecured connection (e.g., HTTP rather than HTTPS). The verbiage suggests the affected components are images.
In Chrome, this setting is enabled for me (by default, I believe):
It was insecure image links in a few of the app’s icons. The My Apps page was affected since it shows all the icons. I changed the image links to HTTPS and the warning cleared.