Mac users not able to “see” jpeg graphic

Why can’t some Mac users see the menu bars (which are jpeg images) on the web site I manage? Many Mac users with machines of very recent origin as well as older machines, report not being able to see the menu on the front page of our website and thus are unable to navigate the site. Has anyone else experienced this problem or heard of it? If so, how is it solved? If you need to see the page and look at the source code, the URL is:
www.grand-canyon.edu

Thanks in advance for any input youcan give me.

Charles Maxson