How can I customize the icons of the folders that are in the dock in Snow Leopard?
The icons in the Dock are the same as those used for the folders themselves. Some folders have custom icons by default, based on their name and location, but you cannot do that yourself.
Replace the icon of a file or folder manually as described here.
Make sure to select Display as: Folder in the Dock folder's context-menu.
Candybar will let you change most any icon in your system. Even the ones in the Dock.