It's no different from changing any other desktop icon you just need to...
1. Select the icon you want to change your ipod to (on the desktop or in a folder).
2. Choose "Get Info" from the File Menu, or click command-I.
3. Click on the icon in the upper left of the info window.
4. Choose "Copy" from the Edit menu (or type command-C).
5. Select your iPod on the desktop and "Get Info" for it.
6. Click on the icon in the upper left of the info window.
7. Select "Paste" (command-V) from the Edit Menu.