Newsstand icons are unique.
Rather than the traditional square app icon, your Newsstand cover icon is rectangular and can be either landscape or portrait orientation.
Newsstand’s intent is for portrait orientation to be used for magazines and landscape to be used for newspapers.
When you use portrait orientation, iOS automatically adds the appearance of multiple pages to the right edge and a shadow that suggests depth to the left edge of your cover.
Similarly, when you use landscape orientation it gives the appearance of a fold at the bottom edge, much like a traditional print paper. Stacked pages will also be on the left and on top of your cover.
Whether you choose portrait of landscape orientation for your icon, the longest edge should be 1024 pixels. The aspect ratio of all Newsstand icons should be between 1:2 and 2:1.
So if you choose portrait orientation, the long edge or height would be 1024 pixels and the short edge or width can be a maximum of 512 pixels.
Now if you choose landscape orientation, the long edge or width should be 1024 pixels and the short edge or height should be a maximum of 512 pixels.
You only need to submit one high-resolution image and iOS will automatically scale it to all the appropriate sizes and formats necessary everywhere your icon is used.
All Newsstand icons must be flat and have 90º corners. Do not add any perspective to your Newsstand icon. You should also save it as a png file.
If you use Photoshop, it would be a good idea to create a reusable PSD file with a layer dedicated to your consistent brand element(s) and a background layer that contains your variable, issue-specific content.
Need more detailed info? Check out the documentation in the iOS Developer Library
Photo of Apple Newsstand courtesy of The Tech Block