You can make an image backup using Win32 Disk Imager or other similar software like it.
If you want to make a backup image of an operational system, I would advise you to use
dd. An important thing to consider here is to shut down all running processes before you make the backup using
dd so as to make sure nothing is being written/read from the SD card. Common processes to shut down would be Apache/Nginx, MySQL server and others like it.
I personally use the following command to run a backup of the SD card every night using cron and copy it to a mounted External HDD
sudo dd if=/dev/mmcblk0 bs=4096 | pv -s 7761M | sudo dd bs=4096 of=/media/NASDRIVE/backup.img
In the above code,
/dev/mmcblk0 is the SD card and
/media/NASDRIVE/backup.img is the External HDD path to which
dd writes the backup image. You may omit the
pv -s 7761M part of the code but it shows a nice progress bar while backup is running.