I am using LXD (“Linux Container”) a container “hypervisor” on Ubuntu Linux serer. How do I list all available VM images for installation in the LXD image store?

lxd is a system-wide daemon and lxd is a command line client to talk with the lxd daemon. So, you need to use the lxc command to list images in the LXD store. The syntax is as follows:

lxc image list images:
Sample outputs:

You can use the grep command or more command as follows:
lxc image list images: | more
lxc image list images: | grep -i 'debian'
Sample outputs:

You can apply filter as follows to just show alpine Linux vm images:
lxc image list images: 'alpine'

You can now create a VM as follows:
lxc launch images:alpine/3.5/amd64 alpine-www
lxc launch images:centos/7/amd64 cenots-db
lxc list