If the https:// is crossed out and in red, there is a SERIOUS problem. Click the lock icon and tell us more. However, if it's grey, it's okay.
+1, not sure what OP was seeing. Here are some screenshots to make the "external images" thing clear:
When I connect to my router, for example, it shows the red crossed out HTTPS because the router has an invalid self generated certificate.
One other reason I think it could show up potentially in your case is if you use OpenDNS or similar - when OpenDNS blocks some HTTPS content, the certificate is invalid because it is for the OpenDNS domain, and not the domain that was expected by the browser when fetching the resource. That could be what is happening to you.