On Dec. 30, 1930, the first-ever photo of the Earth's curvature was taken by Lieutenant Colonel Albert William Stevens, who was an officer in the U.S. Army Air Corps and an aerial photographer.
This photo is one Williams took over South Dakota in 1936 during a record-breaking balloon flight, which also shows Earth's curvature.
(Image credit: National Geographic/Albert William Stevens 1930)
I don't think the flat Earth idea is nearly as widely believed as it appears. I think most of the proponents are just trolls.