I just went to zillow.com (looking at 48237) to see what would happen on my Mac and both the "slide show" and the video worked fine. Are you up to date with all of the updates available for your applications and Mac OS?
If you would like give us a specific link or two we could try them and verify if they work correctly.
I think it's a settings issue. I checked the link below in Firefox and it worked no prob. Even the splash page of a house has some missing pics:
Photo on Zillow
- One Nation Under A Groove -