[cd34]

flicker on and then disappear... are you using CSS mouseovers?

Do you set the expire time specifically on images? or are you using a php script to send the images to the surfer? Some CMSs do this which sometimes affects the expire time causing a refresh, but generally shouldn't cause a problem. Do you block on the empty referrer? Some privacy software might erase the referrer, but then I would suspect the image to have never shown up.

If your CSS is not set properly, its possible that another element is being rendered over it that whatever browser you use to test isn't seeing. Nested tables that might have a width on a table cell set incorrectly.

Many possibilities and perhaps the Mac is just being more standards compliant. I know that there are sites that older versions of firefox would not render properly because they were strictly adhering to standards. if someone had a 0 width box, then lo and behold nothing in that box will be visible -- while safari and IE would say, well, that's probably not what the author intended, we'll display it anyhow.