If you need to create icons for an application on a Mac OS X system, Apple provides a small application named "Icon Composer". If you have the Xcode tools installed, this application is in the
/Developer/Applications/Utilites/Icon Composer folder on your system. (Choose Macintosh HD, then Developer, Applications, Utilities, and then the Icon Composer application.) More to the point, this utility program helps you create icons in Apple's "icns" format from icon images in other formats.
To use the tool it's preferred that you supply images at four different resolutions: 16x16, 32x32, 48x48, and 128x128. But, you can create a
.icns file with as little as one image. Apple's help on this subject states: "If you give IconComposer only one image, it should be optimized to look best at 128x128 pixels. This ensures it looks goods when displayed at the maximum resolution of 128x128 pixels, and looks acceptable when scaled down to the smaller resolutions."
Regarding image formats you can supply, the help system also states: "IconComposer can create icons from the following image formats: GIF, JPEG, PDF, PICT, PNG, TIFF, and any of the QuickTime-supported image formats."