Adding in an exception for the web images is not as simple as just adding in a reference. There is logic in the code that deals with the image type, just adding in the one you would like to also include will not allow it to work correctly.
If you can upload the images directly from the camera with the same page in IE then it indicates that the image type is not what is being restricted. When you took off the web images restrictions does the page function the same in IE as it does in FireFox?
In order to be able to use the images with DFP successfully you may need to copy them to your hard drive and rename them properly if the names are a problem.