dd (Disk Dump)

# read the block device and hand the output to xz so as to compress it on the fly
sudo dd if=/dev/sdb | xz > screenly.xz

# decompress the image and hand it off to dd to write to the block device
xzcat screenly.xz | sudo dd of=/dev/sdb

# note: xzcat is an alias to xz --decompress --stdout