[grn]

My explanation is as follows: the original image uses a checkerboard pattern. When you make a thumbnail it removes some rows and columns to make the image smaller. When you remove all even rows and columns of a checkerboard the fields that are left are of the same color. Similarly when you remove odd rows and columns.

In this case black and white fields of a checkerboard correspond to pear and apple pixels, so you are left with one of the images.