As this does not always work as expected - modifying the image does work, but using the image on an usb-stick does not, I will provide an update on how to accomplish that.
Download an official Ubuntu ISO image. And run the follownig commands:
# mkdir -p /media/ubuntu
# mount -o loop Ubuntu.iso /media/ubuntu
Now we copy the contents of the ISO to a new directory
# cd /media
# tar -cvf - ubuntu | cd (/var/tmp/ && tar -xf - )
In this directory (var/tmp/ubuntu) we can make all the necessary modifications we need.
# cd /var/tmp/ubuntu
# mkisofs -o ../remastered.iso -b isolinux/isolinux.bin -c isolinux/boot.cat
-no-emul-boot -boot-load-size 4 -boot-info-table -J -R -V UBUNTU .
The created ISO does not work yet. You still need to execute
# isohybrid /var/tmp/remastered.iso
Now you can use unetbootin or start up disk creator to burn the image.
