Where to find an original Android image? Possible Duplicate:
  Where can I find stock or custom ROMs for my Android device?  




I have found many custom ROMs for download. But now I want to use the official ROM for my phone. Is there any way to do it without a NAndroid backup?
Download ROM Manager and go to the "Stock Images" section of "Download ROMs". 

You should always do a Nandroid backup when messing with system files. There's always a risk of something going wrong. Also when installing new ROMs (whether custom or stock), it's recommended that you do a clean wipe of the system before installing.