backup the ubuntu-16.04.3-desktop-amd64.iso image

Boot to USB / liveCD and you can make an image of the disk using dd. You will need a writable disk or network share with enough storage to hold the entire disk. You can even use it to duplicate the system. Just dd the image to a new disk. You'll obviously need to change /etc/ostname and /etc/hosts and you're good to go.

If the storage is removable then you can shove it in another system and make an image of the storage that way

Hi,

Thanks for the information provided. Actually i have tried backup using dd command to backup my image but the image size is showing total hard disk size , so we are not able to proceed with this. If you can help me how to proceed further it would be helpful for us. or else is there any tool to backup the customized image to serve my prupose.

Thanks,
Raghuram

It will do. dd goes from the start to the end of the device.

You could pipe dd into gzip and compress the data

Could you share the command how to do back up using dd command

dd if=/dev/sda | gzip -9 > /backup/media/image.iso.gz

Something like that

Or even

dd if=/dev/sda bs=1MB | gzip -9 > /backup/media/image.iso.gz

Your question is somewhat contradictory, and I am not sure what you really want to achieve.

ubuntu-16.04.3-desktop-amd64.iso is the installer image file that can downloaded from http://releases.ubuntu.com/16.04.3/ubuntu-16.04.3-desktop-amd64.iso so there should not be any need to make a backup copy of it.

I assume that you want to create a new installer based on a system that you have already set up. For doing this dd is definitely not the right program, because it creates a copy of your system and not a new installer kit.

There were tools named "remastersys" or "Ubuntu customization kit" available for doing this, but these have been abandoned.

I suggest that you check https://alternativeto.net/software/remastersys/ and try some of the listed current alternatives (e.g. systemback or customizer)