Security Certificate Problem // Problem with images not loading

Hi everyone,
I run http://www.rsihealer.com, and am having a problem in IE/Firefox. Some users are reporting they get a message loading at my site saying there is a security certificate error and that users need to log in with name and password.

I am having a problem that I think might be related with images. They upload okay and show up in media library but somehow when I insert them in a post it tries to load them as an https:// file, and then they only show up sporadically when you load the page. Otherwise they show up as a broken image with a ? in a square…

Any suggestions on what to fix? Is this a problem with the server or a setting/plugin in WP?

Thanks!

Amy // http://www.rsihealer.com