Summarized from https://stgraber.org/2016/03/19/lxd-2-0-your-first-lxd-container-312/.
Interestingly, the LXD command line client is named.... lxc!
lxc image list ubuntu: # ubuntu: is officially supported image source
lxc image list images: # images: is an unsupported source
lxc image alias list images: # lists user-friendly names