If you're looking for a traditional picture of American life, often called "Americana," I would recommend Normal Rockwell's art. For years he was the illustrator of the Saturday Evening Post magazine covers, and he depicted the best of American life in the 1940s and '50s. While the settings may be different for a modern world, I think you'll find the human elements are still the same.
I think the above posts gives you some great ideas on what type of pictures to look for to represent American culture. I think one thing I think of first is a picture of a Fourth of July Celebration.
If you are looking to represent America today, you need to think of that which describes out current America: economic woes, the TEA party, Green energy efforts, Obama, freedom, instant gratification, technology.
I recommend doing a google images search.
If you are looking for significant images that reflect our culture over the ages, think about the significant moments over our nation's history: the Revolutionary War, Declaration of Independence, Constitution, different American flags, Civil War, industrial revolution, Gold Rush, pioneers, women's rights movement, civil rights movement, flappers, prohibition, Great Depression, WWI, WWII, sports, and 9/11. You could put together a great collage with a few clicks of your mouse.
American culture encompasses traditions, ideals, customs, beliefs, values, arts, folklore and innovations developed both domestically and imported via colonization and immigration from the British Isles. Prevalent ideas and ideals which evolved domestically such as important national holidays, uniquely American sports, proud military tradition, and innovations in the arts and entertainment give a strong sense of national pride among the population as a whole.