Initial Helpers

Some helpful, quality starting points:

• "25 of the Most Iconic Photographs," from CNN.
• There's a TIME book called, 100 Photos That Changed the World that you can probably find in your local library. This Facebook page shows thumbnails of those photos—but you'd have to dig around to find out what the photos' titles were, or any of the details about them.
• This Huffington Post article, "We've All Seen These Iconic Photos. Here's the Remarkable Footage That Brings Them to Life" is interesting because it plays into what Morris talks about, which is our desire to question or not trust "the truth" of a photograph. In the article you'll see many examples of iconic photos, but below them are GIFs that capture moments that led up to or followed the picture being shot.
• You can probably easily find some photography books, historical photography books, or famous photography books—it would be a great way of finding key images. 

The sources above should help, but let's keep in mind the following:

• The photo needs to be one everyone—or most—would recognize. That's the point. It's an iconic, famous image. 
• The photo doesn't have to be historical. It could be a famous celebrity or sports photo (I can think of some already that people would know). It could be a famous political photo. A famous nature photo that everyone would recognize. Again, there are lots of subcategories here, but it has to be a famous photo.
• So, beyond that you should pick a photo you can actually work with. Does it have a story behind it, and can you find that story from a legitimate source? Is the story interesting enough to you? Can you find the photographer, and is there anything interesting you can find out about them? You might have a great photo, but if you cannot find out anything about it, then move on to something that will work—and that will be interesting for you and your readers. 
• Also, keep in mind there's a difference between a famous "event" and a famous photograph. Finding a random photo of something big that happened isn't quite the same. It's the image—and the fact that the image is what became famous.