Open your apps folder and select itunes. Right click, and select Show Package Contents. Open Contents, then Resources, then find iTunes.icns. Replace that file with your desired icon, making sure to rename it iTunes.icns. Close all the windows and you're done!
Edit: It has to be in .icns format, so if it's a jpg or png then you'll have to save it as .icns.
Could you post a screenshot of your iTunes' Resources folder, with the iTunes.icns icon in the shot plz? That would answer a few questions.Unfortunately, this simply does not work. Mac Pro 2008, OS X 10.6.4. Anyone know why?
The only thing I can think of is clearing your icon cache. There has to be a way to do it manually, but I don't know how. There are some apps that can do it, such as Snow Leopard Cache Cleaner, OnyX, MacPilot and others. If this doesn't work...I give up.
Open the Applications folder, then Cmd+I on the ITunes icon. Click the lock in the lower right hand corner and enter your password. Give the Everyone group Read & Write access, then follow the instructions above. Once you've changed your icon, change your permissions back.