Original by Sharon Berg here - http://www.pbase.com/sharonjoy/image/101166951

OK Sharon - first off, the image isn't crooked!

You can't look at a field and say it's crooked because the edge of the field is not perectly horizontal - fields can run diagonally - unlike water horizons.

If you take a look at this graphic, you will see the grid superimposed that shows all the trees to be vertical - which is the only thing you have to go on.

You can turn the grid on and off in Photoshop by using CTRL and '