You can do a limited amount with:
Mac 101: Change your icons - http://support.apple.com/kb/HT2493
Icons are no longer stored in a files resource fork. They are stored separately in .icns files and linked or "attached" to files.
Some icons are system level. They are stored in a system file which is not available to the end-user and link with relevant file types (eg Application and System folder icons). You cannot change these.
Other non system files (folders, files, applications) can have their icons changed by the end user. Icons are stored in self-contained files with the .icns extension. You can search for these .icns files and then open one in Iconographer."
OSX icon locations - http://discussions.apple.com/message/6526923
Where is a custom document icon stored? - http://forums.macosxhints.com/showthread.php?t=75147
Mac OS X: How to Use Custom Icons - http://support.apple.com/kb/TA20788