...things like photo images or links are connecting in an insecure manor to a remote server. It can be something as minor as an out of date security certificate for the server that houses that image. If you left click on the little lock it will tell you what triggered the security warning.

This is rather common with the security levels of modern browsers. It is not necessarily a cause for concern.

as you probably know, we don’t have a way for members to upload images for the purposes of including them in posts. Instead, members include images in posts by linking to them elsewhere on the internet.

When you see a DU page displayed as “not secure” in your browser, it is almost certainly because somewhere in the thread you’re looking at, someone has linked to an image which has a URL that starts “http” instead of “https”. The browser then considers the page to be “partially secure” because of that element.

This is an issue that has actually declined in recent years — as more and more websites have switched to SSL, people are less likely to come across “http” images. We used to be concerned that it would be confusing for our members if we disallowed linking to them, but since most images are “https” these days, we may try to come up with a way to block “http” ones going forward.

Nevertheless, DU does use an SSL certificate and all data that is served by us — including pages where you enter data such as the login and post pages — are “https” pages.

I've had it happen in the last few days.