The main theme of the poem "A Photograph" by Shirley Toulson is the transient nature of life, the enduring power of memories, and the impact of time on our perception of the past. It reflects on how memories persist even in the face of loss, the bitter-sweetness of nostalgia, and the inevitability of time and change. The poem invites introspection on the fleetingness of moments and the importance of cherishing memories amidst the transient nature of life.