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Explanation
This comic features aliens observing and commenting on human behavior. One alien notes that humans behave as though they have conscious, thoughtful possession of free will, but that you can manipulate their brains "with the most trivial actions."
The alien then demonstrates: showing a human a collage of photos from their life experiences causes a profound emotional reaction. The twist is that this supposedly manipulative stimulus is exactly what humans already do voluntarily -- looking at photo albums. When described clinically as "the same photos displayed one at a time with fade-ins and fade-outs between them, plus music," it is revealed to be a simple slideshow.
The final panels show the alien reacting with awe: "The days, the years -- it's like they're just so fast," and the other alien notes "It's like a completely different entity." The joke is that the aliens set out to demonstrate how easily humans can be manipulated, only to be emotionally manipulated themselves by the same simple slideshow. It satirizes both the reductionist view of human consciousness (that emotions are just mechanical responses to stimuli) and the genuine power of nostalgia and memory. Even beings who intellectually understand the "trick" are not immune to its emotional impact.