Aha-Soft provides a collection of ready-made royalty-free icons conforming to Apple's guidelines for toolbars, navigation bars and tab bars. App Tab Bar Icons for iPhone offer a simple way to design great-looking navigation bars, toolbars, and tab bars in iPhone, iPad, and iPod apps. 120 unique application icons are included in all standard sizes of 20x20, 30x30, 40x40, and 60x60 pixels. Each icon is supplied in black and white versions. Raster PNG and PSD files and scalable vector AI, SVG, and PDF sources are supplied. Different bundles sets available for iPhone development, i.e icons for iPad, icons for iOS developers.
You can buy bundled icon sets to suit your needs or individually for $5 each. All icons offered are royalty free. Check it HERE (http://www.aha-soft.com/stock-icons/high-resolution-app-tab-bar-icons.htm?ref=ch) for your bundled set or individual icon.
There are different icons, each with different sizes and for specific purposes. There are different devices running IOS. This means that application designers to know when and how to sort icon is.