I'm still a little lost here. I get the general intuition, but what is actually going from the tree to the point on the image? Is it a photon? Is it a ray of light? Where does it come from?

The light will hit the 3D object(the objects in the world) and bounces the light off it, parts of the light will though the camera pinhole to the image. when the light impacts the objects may be reflected or absorbed(modifies the light) to creating color, brightness...