Add image for tests.

The image was downloaded on 2013-05-31 from https://secure.flickr.com/photos/fanglanhsu/8902217876/sizes/m/in/photostream/.
The license is Creative Commons and allows modifications.
The author of this image is 'fanglan' from flickr.com (https://secure.flickr.com/photos/fanglanhsu/)
Add tests for generate_thumbnail()