The way you change any icon is by first selecting an icon, pressing Command+i, clicking on it's icon in the upper left-hand corner, and pressing Command+c. You then click on the icon you want to change, press Command+i, click on it's icon in the upper left-hand corner, and press Command+v. You can do the same thing with a picture in preview or a graphic in Photoshop by copying it to your clipboard and pasting it onto the icon like the later method.