Hi Quang,
Actually ALL members of the AIF (Australian Imperial Forces) were volunteers, not just the Light Horse.
Australia was one of the very few nations in WW1 that fielded an army made up entirely of volunteers. Most other nations introduced conscription at some point during the war.
I've checked the reference photos you supplied in one of your previous replies.
These are studio portraits taken in Australia in late 1914/ early 1915, prior to the Light Horse embarking for Egypt, or taken in Cairo before the Light Horse were sent to Gallipoli.
Cheers