This week's theme is 20th Century inventions.
I think these stamps will do. From a series of USA stamps on Building a nation.
This is a group of four stamps from a larger set. Most of the photographs were taken by Lewis Hine, a famous photographer. According to Wikipedia, "Lewis Wickes Hine (September 26, 1874 – November 3, 1940) was an American sociologist and photographer. Hine used his camera as a tool for social reform. His photographs were instrumental in changing the child labor laws in the United States."
More images can be found at http://www.loc.gov/pictures/collection/nclc/.
Visit Sunday Stamps to see who else is joining in!