These images come from

http://upload.wikimedia.org/wikipedia/commons/9/9a/PNG_transparency_demonstration_2.png

http://fc07.deviantart.net/fs71/f/2012/074/2/4/flores_png_by_lacomegalletasmarii-d4stwqn.png

They were obtained by a Google search for images which are "free to
use, share or modify, even commercially".

