so he actually asked how to change them, not why...
inoob, so go to the application you want to change the icon of using cyberduck, and under each application, there is usually something that says "icon.png." this is the file you want to swap with the new file.
remember to save the old icon somewhere before you delete it in case you want to change it back. also, each icon you use for replacement must be 64 x 64 and on a TRANSPARENT background. you will also have to change the name to match the old one. so in the case of the icon for camera, you will look for "icon-camera.png" and replace it will a file that you have renamed "icon-camera.png."
hope this helps. let me know if you need more help.