Return the reference to img obj in each Function.

With this two or more operations can be squashed together i a single line.
For example, following 3 lines
   $image->scale(50);
   $image->resizeToHeight(500);
   $image->save('picture2.jpg');
can now be replaced with following single line.
   $image->scale(50)->resizeToHeight(500)->save('picture2.jpg');