Folder Icon Creator is an app to customize the icons on your Mac.

The program comes pre-bundled with hundreds of icons that you can use to create custom folder icons and custom file icons on your Mac. Not only are there hundreds of icons included with the program, but you can create a library using ANY PNG, GIF, and JPG images. Just drag and drop images from your desktop into one of the custom libraries, and convert them to icons easily using this program. Once they are in your library, you can customize the icons of folders and files.

Simple and easy to use. You can change a batch of icons all at once. You can import PNG images from anywhere (including the web), and they will automatically be converted to icons for your use.