I had the same problem once and had to replace mys images by gauge to solve it.
But of course in your case i guess you only want to use images.
Maybe it's due to te way you're accessing your app via the web browser , are you doing it with the IE - Plugin or the AJAX client?
For me, the IE Plugin version tries to find external document on the client side (for example when you trigger a "launch application" action) whereas the AJAX version looks for the extrenal document on the server side.
Maybe it's the same thing for images.