In Safari, after the page has loaded, open the Activity window (under the Window menu).
What does it say there?
I agree with Fryke that it doesn't seem like this could be plugin-related.
Have you tried control-clicking on the images and opening them in new windows/tabs? Knowing whether that works could help in diagnosis.
I once had a somewhat similar problem that I nailed down to a bad DNS server. Check to see if your Mac and Windows machines are using the same DNS servers. In OS X, you can find this by going to System Preferences > Network, and then double-clicking your active network mode (probably Ethernet or Airport). I have to be honest, it doesn't sound exactly like your problem
when it happened to me, it wasn't just that I couldn't get images a lot, but also pages would refuse to load just as often. But I'm running out of ideas....
It's likely that your Mac gets its DNS server(s) directly from your router using DHCP. You could try setting it manually and mess around with it. Your ISP probably has more than one DNS server, otherwise you could run some tests with
OpenDNS.
However, if your Mac and Windows machines have the same DNS setup, maybe that's not worth trying.