The easiest way to create a bootable USB key or bootable SD card from Ubuntu is to use a program already installed called gnome-disk-utility (can be found by searching Disks in your applications).
It's usually used to partition/format drives, but can also be used to flash disk images if you go in the top right corner menu after selecting the drive on which you want to flash the image :
Select "Restore Disk Image..." then select the disk image you want to flash and click on "Start Restoring...".
That's it ! Very simple.
