Why my app description images are so blurry on the app store?

I have submitted and got approved my app. I have uploaded crisp and sharp high quality and resolution (960 x 640) PNG description images via itunes connect. They are still crisp and sharp when viewing through the browser at itunes connect ->manage apps ->my app->edit开发者_如何学编程 uploads...

The problem I'm facing is that they become so blurry when viewing them on app store with mac via itunes store. It looks like the images were saved as jpeg files with 60-70 quality. What am I missing? Have you experienced that before? I do not have a paranoya about the perfect quality but the quality of my description images are too low. My app name is Green Pig.

Maybe there are some tricks and hints (other image profiles), color spaces or etc., that could bring a better quality of description images?

UPDATE. Those images are resized, compressed or in other way modified. IMHO that is done to reduce the size of the image. Are there any information how to bypass that optimization? What is the limit size of the description image?

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After uploading 480 x 320 images, the quality has increased. It's not 100% sharp and crisp as my original ones but looks much better now: something between 80% - 90% quality. Those 480 x 320 uploaded images were the size of 119 - 188 KB, however they were still compressed. Hm, maybe it doesn't matter, maybe all images goes through apple image size butcher :)