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dd if=/dev/sdb of=imagem.img bs=1MB

of = output
if = input
bs = Block Size

example (tested):

# to make an image
sudo dd if=/dev/mmcblk0 of=~/Desktop/image.iso bs=1MB

# to write the image to the sd card 
unmount all partitions (mmcblk0p*) and:
sudo dd of=/dev/mmcblk0 if=~/Desktop/image.iso bs=1MB


# ps.: To view all mounted partition type:
df -h

# for further explanation see:
http://elinux.org/RPi_Easy_SD_Card_Setup
https://plus.google.com/u/0/109827810255694125753/posts/Xddvy9T5kqt