Replace rails.png for indexed-color test

We do not have the rights to distribute rails.png, so this commit
replaces it with a free variant. The image we are using,
data/images/indexed_color.png, was retrieved from:

http://en.wikipedia.org/wiki/File:Adaptative_8bits_palette_sample_image.png

and according to that page is in the public domain.

Closes #481.