If you are running launcher pro, you can put custom icons in the docks simply by long pressing an icon->change icon->custom icons then simply pick out a picture in your gallery. If you go to google images and type in "superman" I'm sure there will be tons of images that you can choose from.
If you also have ADWLauncher EX, you can change the icons on the homescreens by longpressing on the icon->edit->press the picture of the existing icon in the window that pops up->select picture->gallery. Then simply select a picture you like. You can also place square tiles behind the icons and choose what color to make them (if you wanted a red/blue theme to go along with it). This is the easiest way I know of without actually making your own icons from scratch.
Hopefully, I didn't just waste your time and post a bunch of stuff that you already knew lol. I hope this helps you.