Final answer:
Matthew Brady's greatest contribution to photojournalism was documenting the American Civil War, which included creating a portable darkroom to develop glass plate photographs in the field and bringing the stark reality of war to the public through his exhibitions.
Step-by-step explanation:
Matthew Brady's greatest contribution to photojournalism is documenting the American Civil War. By inventing a "portable darkroom" and dispatching assistants such as Gardner into the field, Brady was able to capture the aftermath of battles like never before. His exhibition, 'The Dead of Antietam,' showcased in his New York gallery, brought the brutal reality of war to the public. Brady's work marked a significant moment in photojournalism, highlighting the impact of visual media in storytelling and how it can influence public perception. His body of work provided an intimate look at the human costs of war, much in the way modern photojournalists capture current events, earning him the title of the 'Father of modern photojournalism'.