How can I change an application icon in Mac OS X?

• Select the application file in Finder.
• Open the Info window for that file (File > Get Info, or ⌘I)
• Click the icon in that window (a blue border will appear).
• Paste in a new icon.

Any image you can open in Preview can be used as an icon: open it in Preview, select it, and copy it. That will put an image on the clipboard that includes a format that can be pasted as an application icon.

1. Right Click on app in Applications folder
2. Click on Show Package Contents
3. Go to Contents > Resources
4. Check for icon file in the info.plist file located in Contents folder.
5. Replace the icon file (icns) file with the desired icns file.

According to macosxhints.com, it seems that in 10.6 the icons for standard Mac OS X applications (or even Apple software in general) might no longer be easily changed, as the application folders are kind of read-only.

One could of course change those access rights, but changing the package contents might break the Code Signing's signature for that application. And if the signature becomes invalid, then applications might no longer be allowed to access the keychain, will no longer be permanently allowed an exception in the firewall if it's known to check its own integrity (known to have caused trouble for configd, mDNSResponder and racoon), or might cause trouble when using software update.

(Above, might indicates that I am not sure. Some quick tests changing the iTunes, Safari and Activity Monitor icons did not break the Code Signing, though for some other, yet unknown reason the firewall might repeatedly ask Do you want the application “iTunes.app” to accept incoming network connections? For more details see If Mac code signing is tampered with, what might fail?)