Shortnote

This page contains a reference for the image sizes found in the various stock iOS bundles. The sizes vary between devices and the page is organised by application and device type. Using Ctrl+F is advised to search the document for what you are looking for.

A common trick is to replace some springboard items with transparent pictures of the same size. This will eliminate some springboard elements that you do not want to see (eg: dock background, etc…). Also, you have to realise that the larger the image is in size (bytes), the more pressure it will put on your iOS device: an image of 600KB will take more time to load than an image of 3KB and may consequentially occupy more RAM when being cached in memory.

The images on the iOS device are optimized using Apple's proprietary addition to the PNG file format called Xcode CGBI. You can decode them using pngcrush on your Mac provided that you have corresponding XCode and iOS framework installed, found at /Developer/Platforms/iPhoneOS.platform/Developer/usr/bin/pngcrush (which is an Apple modified version of pngcrush):

/Developer/Platforms/iPhoneOS.platform/Developer/usr/bin/pngcrush -revert-iphone-optimizations apple_coded.png decoded.png

Due to the former, it is highly advisable that once you have a theme ready, to use pngcrush for all your PNG files on your mac:

find image_folder/ -name *.png -exec /Developer/Platforms/iPhoneOS.platform/Developer/usr/bin/pngcrush -brute -ow '{}' \;

which will optimize the PNGs and reduce them.

All the images are found at /System/Library/CoreServices/SpringBoard.app and this list includes some third-party apps as well such as Backgrounder.