Change the /tmp directory (or a directory where you would like to store the /image.data file from the mksysb image) and restore the /image.data file from the mksysb:
# cd /tmp
# restore -xqvf [/location/of/mksysb/file] ./image.data
If you want to list the files in a mksysb image first, you can run the following command:
# restore -Tqvf [/location/of/mksysb/file]