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It is a requirement of iPhone and iPod Touch applications that they have an icon of appropriate dimensions in order to submit your application to the AppStore. This article looks at the different sized icons required, and also how to create icons for use by Tab Bar applications.

It's best to start big with your icon as you'll want a 512 by 512 pixel image to use when submitting your application, and you can always resample this to smaller sizes without too much of a problem. When I was writing Linnaeus Lite / Pro this was something that I didn't realise as I'd not actually signed up for an Apple Developer license at that time which created an "interesting" dilemma of whether I should restart my icon from scratch. When submitting your large icon to iTunes Connect they state the following:

A large version of the application icon. This will appear in the App Store. It must be flat artwork, without rounded corners, that has not been scaled up, that is at least 512 x 512 pixels and that is at least 72 DPI.

It is important that you follow these guidelines as it is well known that Apple are quite strict when it comes to approving applications and you don't want to give them a reason to reject it. When you've designed your icon and have saved it ready for submitting you can also save a copy as a PNG or JPEG which can be used when trying out your application so that it gets an icon in iTunes. To achieve this name the file iTunesArtwork (with no file extension). Once you've done this right click on the Project folder for your project in Xcode and from the contextual menu click on "Add" and then "Existing Files...". You will then need to locate your image from the new window that appears and on clicking "Add" you will be presented with another dialogue window. All you need to do in this new window is to click "Copy items into destination group's folder" and then click "Add" again.

For the icon that is displayed on your iPhone or iPod Touch's home screen you'll need to create another copy of your icon. This time scale the image down to 57 x 57 pixels and once again you should make sure it does not already have rounded corners, alpha-transparency, or the shiny part you often see on App Store icons - both of these effects will be applied automatically to your apps icon. Once you've created your smaller icon name it as Icon.png and add it using the same method described previously. One thing worth noting here is that if you size your icon to any other size the iPhone OS will automatically scale it to the correct size.

You'll want to create one more version of your application icon before you're ready as well - this one should be sized at 29 x 29 pixels and will be the one that appears in Spotlight results. The same rules apply to this icon, and this time when you add the icon to your project it should be named Icon-Small.png so that it can be found. It is possible to get away with not having this icon though as if this one isn't present it will scale down the main application icon to fit.

Tab Bar Icons

Another type of icon you're likely to want to create is for the TabBarController which is a View Controller for switching between different views which don't necessarily have to be of the same type. These icons tend to cause a problem though as they have to be created slightly different in order to get them to display correctly. The reason for this is that the Alpha channel of the PNGs will be used to determine which areas should switch colours (between white and blue) depending on whether the tab is active. This gives a nice consistency and expectation to touch applications as they'll have a common UI. If you don't want to create your own there are a number of pre-made icons that you can use.

Icon	Meaning	Name
	Show application-specific bookmarks	Bookmarks
	Show Contacts	Contacts
	Show downloads	Downloads
	Show user-determined favorites	Favorites
	Show content featured by the application	Featured
	Show history of user actions	History
	Show additional tab bar items	More
	Show the most recent item	MostRecent
	Show items most popular with all users	MostViewed
	Show the items accessed by the user within an application-defined period	Recents
	Enter a search mode	Search
	Show the highest-rated items, as determined by the user	TopRated

The above table comes from the Apple developers website and demonstrates the intended style for the icons. Similarly you'll find there are a number of pre-made buttons for use in navigation bars. If you decide you want to make your own then you must remember that the background should be transparent, it should be a alpha-transparency supported PNG, and the image itself should be in black. These icons should be around 30 x 30 pixels and is recommended by Apple that if there is a bevel to them that they follow the same "light source".