You mean the graphic that is on the application in the Finder? I would suggest something like Iconographer X or something similar so that when your app's icon is in the dock, it will actually look good at all sizes.
Other than that, just use any graphics program to make what you think looks right.
I'm sure Iconographer can tell you how to actually insert the icon into your app, but I believe the simplest way is to copy it, and then get info on the app, click on the existing icon, and pasting your new icon there.