The usual method for changing a program's icon in OS X is outlined here. Unfortunately sometimes this method doesn't work because the icon image you are trying to use actually has a generic image icon in it's info box instead of the image itself, such as seen in the two boxes on the right in this image (click for full view):

So I made a little program based on a script I found here that will fix the preview icon for any image. Just double click on iconifier, point it to your image, and voila, the image in the upper left of the info box will match the actual preview image found down below.

This might be entirely unnecessary, and there might be any easier way to do this - but this is all I could figure out right now.

Download: Iconifier.app (13k)