replace test images by images from public domain

The original images were licensed under CC-BY-SA-2.5, considered
non-free by the Debian project
https://commons.wikimedia.org/wiki/File:Phalaenopsis_%28aka%29.jpg
The new files
http://www.public-domain-image.com/free-images/flora-plants/flowers/orchid-flower-pictures/white-and-red-orchid
are licensed under the CC public domain