iPad/iPhone on iOS 5 - images don't always load in Safari, just on my network
Tough one.  Definitely need to see more troubleshooting.  Some places to start:


Find an image that the iPads are having difficulty loading, then open just that image in a separate page in Safari.  Any problem loading and reloading the image on a page by itself?
How many WiFi devices are being served by one base station?  At some point it gets overloaded with sheer volume of requests, even if the data transfer is small.  
Is the WiFi base station doing NAT, possibly leading to a double-NAT setup that will clog the local network with failed connections?  You would still be able to browse the web, but overall network performance would suffer.