I too recall seeing the photograph you are seeking, but as with you, I have been unable to find it with a search this evening of both "Pictures Australia" and the AWM.
From what I can find, the Corryong and Walwa Troops were formed in early 1935, "A" Squadron, 8th Light Horse Regiment.
Without the photograph in question, and its caption, it is difficult to give you an exact date, but if this helps in some way, the following are the dates of the Royal Melbourne Show in which the 8th Light Horse participated in the military events.
Wednesday 26th September 1935. (The Corryong Troop was in the "Four over Hurdles Event.")
Wednesday 23rd & Thursday 24th, September 1936. Trooper Blamley(?) Walwa Troop won the tent pegging event on the 23rd.
Thursday 30th September 1937. 8th LH third in the "four over hurdles event."
Have not found a date for 1938 as yet, but it will be either the last week of September/first week October.
What I have got of the 1938 Military event for the Show is this -
Capt E. G. Palmer
Lieut C. Nason (? might be Mason), Troop Leader.
Sgt H. Sheppard
C. Pearmane (could be Pernaine, have both spellings with initial C.)
Jack Peck (H. J.)
Jack Drury (A. L.)
All "A" Squadron 8th Light Horse. Any names connect with the Corryong/Walwa Troops?
There is also another possibility to the photographs origin, the annual Navy, Army and Air Force competions and Gymkhana held at the Showgrounds, Flemington.
The 16th Annual Competition and Gymkhana was held on Saturday 7th December 1935, the 8th Light Horse again taking third place in the "Four over hurdles event."
What a great pity that those fine Australian Miltary traditions, the Tattoo and the Gymkhana, have fallen by the wayside, was a highlight when I was a youngster and my Dad would take me to those at the showgrounds back in the fifties.