When you capture your WIM initially, you can specify the compression type. Example: Unfortunately, you cannot change the compression level on appends, only the first time. However, you can export images and recompress that way, so this command may be more to your liking: Other command options can be found under the "ImageX Command-Line Options" page of the Windows Preinstallation Environment (Windows PE) User's Guide. This CHM is in the Documentation folder of the WAIK in the Start Menu.