How to use a .iso image as a CD-ROM Repository?

This question has already been answered as part of an answer to a different question: How to make USB drive as local repository

The easiest way to add the image as a software source is to burn it to a CD/DVD. You then need to go to System -> Administration -> Software Sources, click 'Other Software' and click 'Add CD-ROM...'.

If you want to use the ISO image as a software source without burning it, the process will be a bit more complicated. You will need to open a terminal Applications -> Accessories -> Terminal and run these commands:

sudo mkdir /aptoncd-mountpoint

sudo mount /media/USB/aptoncd.iso ~/aptoncd-mountpoint -oloop

sudo apt-cdrom -d=/aptoncd-mountpoint add

(source: http://www.debianhelp.org/node/10486)

Try this

mkdir /media/mountpoint  
mount -t iso9660 -o loop /pathtoiso.iso /media/mountpoint  

then add deb file:///media/mountpoint distro main contrib to /etc/sources.list with text editor and not with some GUI tool, then update from package manager.