This manual was tested on Debian Lenny.

Install helper scripts
<code>
sudo -s
apt-get update
apt-get install live-helper
</code>

Create live system environment
<code>
mkdir -p ~/liveusb/
cd ~/liveusb/
lh_config -b usb-hdd \
        --hostname rescue \
        --packages "python console-setup console-cyrillic libstdc++6 libntfs10 e2fslibs libparted1.8-10" \
        --username root \
        --bootappend-live "locale=ru_RU.UTF-8 keyb=ru quickreboot" \
        --syslinux-timeout 5
</code>

Cyrillic and python are just my own requirements and you can drop it if you want.

Now we will put partimage-ng binary to the-soon-to-be-build liveUSB image :)
<code>
mkdir -p ~/liveusb/config/chroot_local-includes/usr/sbin
cp ~/partimage-ng ~/liveusb/config/chroot_local-includes/usr/sbin
chmod 755 ~/liveusb/config/chroot_local-includes/usr/sbin/partimage-ng
</code>

I also turn off dhcp-client when live system boots
<code>
cp ~/01-disable_service.sh ~/liveusb/config/chroot_local-hooks
chmod 755 ~/liveusb/config/chroot_local-hooks/01-disable_service.sh
</code>

Contents of 01-disable_service.sh
<code>
#!/bin/sh

/bin/echo "I: disable networking"
/usr/sbin/update-rc.d -f networking remove
</code>


Let's build the LiveUSB image now. This will take time, so be patient :)
<code>
cd ~/liveusb/
lh_build
</code>

If everything goes well we will have binary.img file, which should be copied to usb-flash with dd like this
<code>
cd ~/liveusb/
dd if=binary.img of=/dev/sdb
</code>

Now you have a bootable liveUSB Debian with partimage-ng onboard :)

Hope this will help :)