Using "zip -9".
The point is that there is almost nothing on the image except for a partition table and a few MB of files - plus almost a GB of zeroes. And these can be compressed to 20 bytes or so.
So, no magic here unfortunately - the only important thing was to zero out everything before formatting and putting files
Insider tip: That's also how I optimized the main partition images. Simply mounting it, "dd if=/dev/zero of=/mnt/zeroes; rm /mnt/zeroes" will also allow to pack filesystem images efficiently. That reduced the compressed size about 20-50 MB for each image... And of course, using lzma instead of gzip makes a huge difference too.