Replace Medusa image with Alex Schenck's Beowulf image

Replace Medusa image with Alex Schenck's Beowulf image

To clear up licensing issues, change to an image used on the Wikimedia
Commons [1].  The image is copyright Alex Schenck [2], and licensed
under under the terms of the GNU General Public License as published
by the Free Software Foundation [3]; either version 2 of the License
[4], or any later version.

The Medusa image was posted with a:

  © 2009-2014 Rich Townsend

footer [5].  I think showing that image with a link to its original
counts as fair use [6], but using an explicitly licensed work is
better.

[1]: http://commons.wikimedia.org/wiki/File:Beowulf.jpg
[2]: http://en.wikipedia.org/wiki/User:Linuxbeak
[3]: https://fsf.org/
[4]: http://www.gnu.org/licenses/old-licenses/gpl-2.0.html
[5]: http://www.astro.wisc.edu/~townsend/news.php
[6]: http://en.wikipedia.org/wiki/Fair_use