Posted by Ronnie Diaz
Using these steps any image you have can converted to an icon to use on Mac.
This following article is a condensed version of one found on MacInstruct. (see references below)
1) Download and Launch Pic2Icon.
2) Drag and drop image you want to create an icon of to the little window. Once it finishes processing, you should have a tiff file.
3) Download and Launch Icon Composer. It is part of the Apple Developer Tools.
4) Drag the tiff file to the bottom left 128×128 window in Icon Composer. Make sure you select “use scaled version” and “extract mask” until all windows are filled up.
5) Save with .icns extension and that’s it!
Apple Developer Tools (Icon Composer), http://developer.apple.com/technologies/tools/