How to change dock icon when some app has created generic icon images?
You might want to make sure your app is correctly associated (for that you can use the free RCDefaultApp panel).

If you fancy changing icons, you can try the arguably pricey Candybar from Panic inc. which is excellent if you love playing with your icons, but that's all it does, so for 6 Starbuck's Coffee Latte you can do that :p

Please note that if you change an icon, you will probably have to restart the finder (and/or the Dock) for the change to be reflected (Logoff also works).