When you create or update your apps in iTunes Connect, you must upload screenshots that are high-resolution. We require your screenshots as high-resolution images so that your app is optimized for the Retina display.
The requirements for high-resolution images are 960 x 640, 960 x 600, 640 x 960, or 640 x 920 pixels. Images must be at least 72 dpi, in the RGB color space, and the file must be .jpeg, .jpg, .tif, .tiff, or .png. You can update your screenshot files at any time in iTunes Connect.
It might be a little early to mark the end of the iPhone 3GS. Despite being released over two years ago, it continues to be a best selling device for AT&T. It remains a very capable device and a very attractive phone at $0. For AT&T, they also have exclusivity on the iPhone 3GS, providing them with an advantage over both Sprint and Verizon. If you want a free iPhone, AT&T is the only game in town.
With retina display devices out for quite some time, it also makes sense that Apple is simply tightening up the requirements. The lower resolution screenshots on the iPhone 3GS certainly don’t make proper use of the retina display on the iPhone 4/4S. It would seem to make sense that you’d rather iPhone 3GS owners seeing high resolution screenshots, then having owners of a retina display iPhone having to endure 480×320 resolution screenshots.
This does pose an interesting question. When will Apple discontinue the iPhone 3GS?
Source: The Next Web