Note that new kernel images go into /boot/vmlinux. The old is put into /boot/vmlinux.old. Thus you can have a /etc/lilo.conf like:
image=/vmlinuz label=linux root=/dev/hda2 image=/vmlinuz.old label=linux.old root=/dev/hda2
Note also that /vmlinux is a symbolic link into the actual file stored in /boot.