What would be better in Opera !

To be able to test another ISO, booting it from grub, unzip the wished ISO and copy vmlinuz... and rootfs.gz to /boot.
You can rename those files in order to identify them easily and avoid any conflict with already existing files in /boot and don't forget to adapt the paths in grub.cfg (see after).
Then, add an entry in /boot/grub/grub.cfg (be careful at root=(hd0,1), this should be the same as the first entry) :

# My former installation
menuentry "SliTaz GNU/Linux (cooking) (Kernel vmlinuz-3.2-slitaz)" {
        set root=(hd0,1)
        linux /boot/vmlinuz-3.2-slitaz root=/dev/sda1 lang=fr_FR
}
# My ISO test entry
menuentry "SliTaz ISO - My Grub Entry" {
        set root=(hd0,1)
        linux /boot/vmlinuz... root=/dev/null lang=fr-latin1
        initrd /boot/rootfs.gz
}

Note that if your ISO has rootfs1.gz to rootfsN.gz, you should create an only file by concatenating them :

cat rootfz1.gz rootfs2.gz ... rootfsN.gz > root

To know your SliTaz version, perform

cat /etc/slitaz-version

. But if cooking, it don't tell which.