Because light travels through the air in a near perfect straight line.
The light travels from the top of the tree (see image), straight through the pinhole, and straight to the bottom of the image.
Light travels from the bottom of the tree, straight through the pinhole, and straight to the TOP of the image.
Which then makes the image inverted (or upside down).
hope this helps!
Because when u draw it usally just takes off the stress from your mind and puts it on a peice of paper..... and when u draw you´re free u can draw anything that really comes to your mind.