In this other post you can see how to mount an iso image file at the system startup.
Now we can see how to insert into sources.list file the pointing to the debian distribution image file.
Go under /etc/apt
Open sources.list file and add(better if at the end of the file) this:
deb file:/home/you/Desktop/debian_image squeeze main contrib
Now we suppose we have squeeze distribution.
If no, change the name with your distribution name.
At the end, from the command line give:
apt-get update
Bye...
