For all images
- Minimum 72 DPI
- RGB color space
Large app icon
- 1024x1024 (cannot be scaled up, but can be scaled down)
- File format: jpg tif or png
- Keep it "flat" as in the new guidelines. Avoid rounded corners.
3.5" Retina Screenshots
- Portrait: 960x640 or 960x600
- Landscape: 640x960 or 640x920
- File format: jpg or png
4" Retina Sreenshots
iPhone 5/iPod touch (5th gen)
- Portrait: 1136x640 or 1136x600
- Landscape: 640x1136 or 640x1096
- File format: jpg or png
iPad Screenshots
- Portrait: 1024x768, 1024x748, 2048x1536 or 2048x1496
- Landscape: 768x1024, 768x1004, 1536x2048, or 1536x2008
- File format: jpg or png
Do the screenshots on a device and you should be all good to go.
For more details on the content of the screenshots and icons read this document: https://developer.apple.com/library/ios/documentation/LanguagesUtilities/Conceptual/iTunesConnect_Guide/iTunesConnect_Guide.pdf
