https://en.wikipedia.org/wiki/The_Boy_Standing_by_the_Crematory

This one. The cost of any war summarized in one picture.

Malcolm X holding an M2, looking cool as hell

Bad trigger discipline though

It is hard to pick one, but this photo has always stuck with me. That is a picture from the Dust Bowl during the Great Depression.

The first photo of a black hole is the most historically significant "first photo of x" that happened in my life time and that I actually understood its historical significance when it came out. So I'd say that's probably my favourite.

First moon landing. It shows what we are capable of

Free Huey is a great one:

Maybe that time a dove landed on Fidel's shoulder during a speech right after the victory of the Cuban revolution.

Deepwater Horizon sinking in the Gulf of Mexico on April 22, 2010.

It caused an equivalent oil spill of 4.9 million barrels and exposed the surrounding wildlife to toxic materials, covering thousands of animals in oil. The cleanup efforts took years.

A prime example of humans messing up this planet for their own gains.

This one really affected me. It's one of the first images from the surface of Mars. I was quite young, and it clicked in me that other planets actually exists and are out there in space.

OP’s photo is my favorite, so I will have to mention my second favorite (though calling it a “favorite” feels off).

This photo was taken in 2003 in Iraq. This man is comforting his son. They are being held in an American camp. IIRC to this day we don’t know what happened to these two.

I think if I had to explain the last 25 years to a time-traveler, this would be the one photo I would choose.

President Taft riding a water buffalo. Always gives me a chuckle.

The terror of war.

Nobody wins in war, and I hate how angry this photo makes me feel.

Margaret Hamilton standing next to listings of the software that she and her MIT team produced for the Apollo Project.

The self-immolation of Thich Quang Duc never fails to amaze me. It's just unreal.

And Antonio Turok's photograph of the 1991 total eclipse of the sun in Chiapas, Mexico. This one because even when I know what an eclipse is and how does it happen, there's a moment in my head when I think "What if it never ends? What if everything stays like this forever?" I see that instant of terror in this photo.

6-year old Ruby Bridges walking to school.

This picture was always so powerful to me. I think I had that one famous illustrated storybook about Ruby Bridges.

Great post and I really enjoyed the replies. Mine is of Chavez visiting Castro in 2006 in Havanna, as Castro was dying of cancer. Both men were imprisoned for a failed coup but later rose to power (Chavez democratically and Castro by revolutionary liberation of the country), and both men died in a hospital bed in their respective countries in 2013.

The "Pale Blue Dot" image of earth taken by Voyager 1. Carl Sagan pushed to have Voyager take a parting shot before they turned off the camera. The earth is about one pixel in size.

This one always makes me melancholic:

Such a big turning point in time, captured on a photograph.

This is Emir Seyyid Mir Mohammed Alim Khan, the last Emir of Bukhara.

The emirate existed from 1785

This is a pre-WW1 colored photograph (1911) by Prokudin-Gorsky

I have many good ones, but i think my favourite is this one:

It's from the famous victory parade in Moscow in 24.06.1945, Japan and USSR were not at war yet, so in a truly masterful trolling stroke, Japan was invited to parade to celebrate defeat of their own allies. On the photo you can see Japanese army colonel and a navy officer (hat visible partially) congratulate marshal Zhukov while French, British and American officers are waiting in line.

As of why, it never fail to make me smile.

Yury Gagarin and Gina Lollobrigida. First man into space and one of the most famous actresses back then.

Called "Earthrise", taken during the Apollo 8 mission in December 1968. Always tried to imagine what it was like to have had that view IRL.

this portrait of Frederick Douglass—an escaped slave who had become a lauded speaker, writer, and abolitionist agitator—is a striking exception. Northeastern Ohio was a center of abolitionism prior to the Civil War, and Douglass knew that this picture, one of an astonishing number that he commissioned or posed for, would be seen by ardent supporters of his campaign to end slavery. Douglass was an intelligent manager of his public image and likely guided Miller in projecting his intensity and sheer force of character. As a result, this portrait demonstrates that Douglass truly appeared “majestic in his wrath,” as the nineteenth-century feminist Elizabeth Cady Stanton observed.

https://www.artic.edu/artworks/145681/frederick-douglass

Buncha queer people having a costume party at the Institute for Sexual Science in Berlin, which was the world's first queer clinic and where the foundations of what we today call Gender Affirming Care were developed.

Because it's a nice innocent photo of a man walking with nobody on his left side.

The Socialist Fraternal Kiss

Communists defeated fascism.

We won 20th century.

We will win 21st century.

All photos are historical photos. That's a lot to choose from.