The most common cause of this problem is that you haven't properly supported different device resolutions. You need to make sure that all your images are optimized for both retina and non-retina devices, not just for your icons and splash image.
For example, an iPhone 3GS has a screen resolution of 320x480 so a background image may be 320x480, but the retina iPhones have a screen resolution of 640x960 so you need to have an "@2x" version of the image for these devices.
This means that your app must contain myBackgroundImage.png
and myBackgroundImage@2x.png
.
Additionally, just because your images are retina compliant doesn't mean that is all you have to do. If you started with a 320x480 image and scaled it up to 640x960 slapped an @2x on it and called it retina you will be rejected. Make sure all of your graphics are the proper resolution, and are not distorted in any way.