Trim disk image for best compression before distributing
kpartx -av disk.img && mkdir disk && mount /dev/mapper/loop0p1 disk && fstrim -v disk && umount disk && kpartx -d disk.img
fstrim is usually used on SSDs but can also be used to remove useless bits from file system images. This is most helpful if the compressed disk image is intended to be distributed, since it will be smaller than an untrimmed compressed image.
Sample Output
disk/: 1.1 GiB (1185869824 bytes) trimmed