Merge pull request #1877 from radarhere/tutorial

Fixed tutorial code and added explanation

docs/handbook/tutorial.rst

@@ -184,11 +184,20 @@ Rolling an image
 
         part1 = image.crop((0, 0, delta, ysize))
         part2 = image.crop((delta, 0, xsize, ysize))
+        part1.load()
+        part2.load()
         image.paste(part2, (0, 0, xsize-delta, ysize))
         image.paste(part1, (xsize-delta, 0, xsize, ysize))
 
         return image
 
+Note that when pasting it back from the :py:meth:`~PIL.Image.Image.crop`
+operation, :py:meth:`~PIL.Image.Image.load` is called first. This is because
+cropping is a lazy operation. If :py:meth:`~PIL.Image.Image.load` was not
+called, then the crop operation would not be performed until the images were
+used in the paste commands. This would mean that ``part1`` would be cropped from
+the version of ``image`` already modified by the first paste.
+
 For more advanced tricks, the paste method can also take a transparency mask as
 an optional argument. In this mask, the value 255 indicates that the pasted
 image is opaque in that position (that is, the pasted image should be used as