My app's icon has a circular logo with a blue background. I included all the required icon sizes for retina, iPad etc. but I read several places that the iTunes Artwork image is "optional but recommended" so I did not include it, thinking it would look fine if any of the others were scaled up a little bit for the devices app store. But, now that it's accepted, when I go into the app store, on the device or iTunes, it looks like they took one of the icons, made a really bad cut out of the circular part to get rid of the backround (there are chunks of white pixels sticking off of the top of it now), then put it over a black background... Why would this be the result?? The icon looks very nice on all the different devices' home screens...
It's optional but recommended because the generated image is going to be undefined in its quality and contents. In your case, they made a bad cut. Take 10 minutes and make a good 512px by 512px image (even if it's just scaled up from your icon) and upload it.