Imagine time travel is invented but uses a lot of energy to open just a dime-sized portal into the past that quickly evaporates—just enough for a camera to see through for one exposure. You can petition for a single photograph of anywhere, any time in the past, so long as you can provide an exact time and place. One photo. What do you want photographed?
@dx this is probably the inspiration for your question, but this book from Stephen Baxter explores lots of possibilities from it https://en.wikipedia.org/wiki/The_Light_of_Other_Days
knowing we all would soon have access to every secret of the past I think I'd try to swipe a photo of the first time my parents met each other, or the first time I met my wife.