To find the vanishing point of the image you need to find one object that is constant throught out image and maintians the same height.

The fence would be ideal follow that to its vanishing point. As Steve stated above, this is a standard picture taken with a 35mm camera. 35mm being the length between the lens and the "film" (ok i know its digital, but tis the same rule :p). Once you know its 35mm then away you go.

Really once the image plane is set its set to the camera, thats why steve said to lock the camera off. I do not think you can adjust the direction of the camera to point to the vanishing point, the original picture was not taken that way.

If you want to model this scene then:

Create a new camera
Attach the picture as an Image plane
Lock all the attributes in the channel box
You can use the camera to check your work and for final render