Insert your flash media
Run the following command to get the current list of devices
diskutil list
Run the diskutil list again and determine the device node assigned to your flash media (e.g. /dev/disk2)
Run
diskutil unmountDisk /dev/diskN
then execute
sudo dd if=/path/to/downloaded.img of=/dev/diskN bs=1m
finally run
diskutil eject /dev/diskN
and remove your flash media when the command completes.