# insert SD and find out where it is mounted
diskutil list
# /dev/disk0
#   #:                       TYPE NAME                    SIZE       IDENTIFIER
#   0:      GUID_partition_scheme                        *250.1 GB   disk0
# ...
# /dev/disk1
#   #:                       TYPE NAME                    SIZE       IDENTIFIER
#   0:     FDisk_partition_scheme                        *15.7 GB    disk1
# ...

sudo dd if=/dev/rdisk1 bs=1m | gzip > ~/Desktop/sdcard.img.gz
# or if you don't want to compress it:
# sudo dd if=/dev/rdisk1 of=~/Desktop/sdcard.img bs=1m

# to restore it, firn unmout the SD
diskutil unmountDisk /dev/disk1
gzip -dc ~/Desktop/sdcard.img.gz | sudo dd of=/dev/rdisk1 bs=1m
# or if it is not compress:
# sudo dd if=~/Desktop/sdcard.img of=/dev/rdisk1 bs=1m