If you have a file with an icon you'd like to use for something else, simply
Get Info on the file in Finder (File menu or select the file and hit Apple-i)
Click on the icon field (it should get a blue halo around the edges)
Copy (Edit menu or Apple-C)
Go to the file/folder/disk you want it stuck on, select it
Get info
Click on icon field
Paste (Apple-V)
You can also paste pictures you've copied from elsewhere.
For a *large* selection of custom icons, visit
http://xicons.com
-s*