The file is named WindowsImageBackup and would be in the root of the External. It needs to be there to be detected by the Repair CD or DVD.
If you want to repair the current install, can you provide more detail about what happened. Have you tried running System Restore from DVD Repair console or Repair CD? Have you made sure Windows 7 is marked Active before running Startup Repair as it must be to repair? Partition - Mark as Active
Do you have a factory Recovery partition or disks which could be used to restore to factory condition?
If not, do you have access to a Windows 7 Install DVD for your version to clean reinstall using the Product Key on COA sticker? Here are tips for getting a purrfect clean reinstall of factory OEM: re-install windows 7