In PIL, since thumbnail() first makes a draft copy of the image, and
then resize it, so thumbnail() can run a lot faster than resize()
because draft() seems a lot faster when resizing from very big images
to small images... (such as the original image is 3000 x 2000, and it
can make a draft really quickly to 375 x 250, and then resize to, say
200 x 133 as a thumbnail)
However, the double resizing probably will make a thumbnail with a
lower quality than if it is directly resizing from the original... as
each resizing involves some approximation.
however, i tried directly using resize() and it is a lot slower.
But looks like if quality is of concern and time is not an issue, then
we can use the resize() to create thumbnails instead.