It looks like that .isz files (Zipped ISO image) are produced by the UltraISO optical disk image management software. In order to to convert ISZ disk image files to standard ISO format you have to use one of the utilities that support ISZ format, for example UltraISO or AnytoISO.
Open ISZ file Open ISO file
A popular disc emulator and authoring program
A disk image converter for Microsoft Windows
Mac version of popular disk image converting program
A handy tool for burning and mastering of optical media
One of the disk image handling utilities and the default program for ISZ images