How to update the USB Ubuntu (ISO) with new software

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http://askubuntu.com – I have Peppermint (based on Ubuntu) running of USB. Its bascially an ISO image that loads using Grub4Dos. Problem is that I each time I load Ubuntu - and want to use i.e. teamviewer or Firefox...I have to install it on the running live system. Since its running from a ISO image...a reboot means the software is gone. I can edit/open the ISO image using WinImage (on XP) - would you know how I can add extra software into this ISO image so next time Ubuntu loads...the new software is on there? Is there some special repositary director where I should copy the software files into and recreate the (HowTos)