If you are comfortable with the command line, you can image a card without any additional software. Run :
diskutil list
identify the disk ( not partition) of your SD card. e .g . disk4 ( not disk4s1)
diskutil unmountDisk / dev/< disk# from diskutil>
e.g . diskutil unmountDisk / dev/ disk4
sudo dd bs= 1m if =< your image file> .img of=/ dev/< disk# from diskutil>
e.g . sudo dd bs= 1m if = 2012 - 12 - 16 - wheezy- raspbian.img of=/ dev/ disk4
( This will take a few minutes)
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