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