Some graphics files won't appear through https

[SacAutos]

This is the weirdest deal. I have a webpage that has a bunch of little GIF files on it. The page is running through https. A couple of the graphics give me the broken image in my web browser but most of them work. If I pop a broken graphic open in a new window, I get a "404 Not Found - nginx" error message. But if I change the URL from https to http for the image, it appears without difficulty. Any ideas as to what the hell is going on here?? Plesk 11.5 at the latest MU on CentOS 5.10