Unless it's wet, Adam Niklewicz's 30-foot tall oak tree mural in Hartford, Connecticut is invisible:
The brick around the tree is weather-treated. The area of the tree drawing is not. When the wall gets wet, the bricks in the tree-drawing area become soaked and dark and the treated bricks do not, causing the image to appear sharply on the wall.
How'd he do it? Niklewicz and his assistants made a huge rubber stencil and then sprayed it with Rust-Oleum. You can see process photos at Twisted Sifter.