how can I download a google compute engine image

There isn't a direct way to download a image or snapshot from GCE, but there's a way to save an image and store it in Google Cloud Storage(GCS) where it can be downloaded. You can use the standard gcimagebundle tool to do this.

You can also create this image using the dd command. On a temporary disk that’s bigger than the one you want to image, run this:

dd if=/dev/disk/by-id/google-diskname of=disk.img bs=5M

You can then run this command to copy it over to GCS:

gsutil cp disk.img gs://bucket/image.img

And later, you can:

gsutil cat gs://bucket/image.img | dd of=/dev/disk/by-id/google-newdisk bs=5M

This will allow you to make an image of your disk and then send it to GCS where you can download it using either the web interface or gsutil.

As an addition to the current answer, you can it directly download a file using SSH / SCP, by adding your public key to the "SSH Keys". Then, using your own terminal :

sheryl:~ sangprabo$ scp [email protected]:/var/www/my-file.tar.gz . 
Enter passphrase for key '/Users/sangprabo/.ssh/id_rsa': 

I prefer that way so I don't need to create a bucket first. CMIIW.