I used this to determine the sizes of images for iOS specific apps.
http://www.appcoda.com/ios-univeral-app-tutorial/
Screen sizes (and thus bg image sizes) should be as follows
for non-retina iPhone (320×480)
for retina iPhone 4 and 4S (640×960)
for retina iPhone 5 (640×1136)
for non-retina iPad (1024×768)
for retina iPad (2048×1536)
Icon sizes should be as follows
for non-retina iPhone – 57×57 pixels for iOS 6 or lower, 60×60 pixels for iOS 7
for retina iPhone – 114×114 pixels for iOS 6 or lower, 120×120 pixels for iOS 7
for non-retina iPad – 72×72 pixels for iOS 6 or lower, 76×76 pixels for iOS 7
for retina iPad – 144×144 pixels for iOS 6 or lower, 156×156 pixels for iOS 7
To fulfill the requirements of universal app, other than launch images and app icons, you also need to create two different sets of application images – one version for iPhone and the other larger version for iPad. To be precise, you need to create four different versions of application images.
You should take a look at the link above to determine how exactly these images should be named and the appropriate sizes for each resolution. Naming images properly and allowing the OS to automatically select the proper one is also possible, and is discussed in detail here
How to support both iPad and iPhone retina graphics in universal apps