I like this question because it challenges basic assumptions and makes you think, but...

If the earth were flat you wouldn't be able to sail around it in your own personal sailboat and end up where you started, which many people have done! Also, brave sailors could prove the earth was flat by sailing to the edge and returning with HD footage of a cliff into the giant abyss. Also, why would the earth be flat while all surrounding planets, which we can see through a telescope, are clearly round?

Nice photos, by the way :)