Download the desired .img file
Open a Terminal (under Utilities)
Run
diskutil list
to get the current list of devices
Insert your flash media
Run
diskutil list
again and determine the device node assigned to your flash media (e.g. /dev/disk2)
Run
diskutil unmountDisk /dev/diskN
Execute
sudo dd if=/path/to/downloaded.img of=/dev/diskN bs=1m
Run diskutil eject /dev/diskN and remove your flash media when the command completes