The isz file (Zipped ISO image) is produced by the UltraISO optical disk image management software.
To transform ISZ disk image files to popular ISO binary format you can use UltraISO, or AnyToISO disk image universal converter. AnyToISO is available for Windows and also for Mac OS X.
You are also able to burn ISZ disk image on optical medium with Alcohol 120%, or mount it to virtual optical drive with Daemon Tools and create .ISO image with CDBurnerXP, ImgBurn etc.