Deleted app ‘?’ icon in Dock

If you want to avoid the Login/Logout, restarting the dock should be enough: killall Dock from the Terminal should be enough.

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).