My gallery pictures are invisible... on some browsers!

Using the basic gallery, I have a problem. My gallery images are sometimes invisible.
They look like they are loading… the bar moves across, the title is there… but no image shows. Yet it works fine on other computers.
For example:
IE on my PC (Windows2000 IE v6): No images
IE on my Mac (OSX10.4.7 IE 5.2.3) OK
IE on another Mac: Same specs: No images
Other PCs and Macs: OK

Are there any known “browser” issues with this gallery?

the site is
Any help appreciated.