Replace Lenna image in tests with a truly free alternative.

The Lenna image used in image processing tests is considered problematic due to
its unclear copyright status. Right now it is considered to be "overlooked" by
the copyright holder Playboy.

A suitable replacement image is already provided in the libav sources which is
licensed under the Expat (MIT) license and thus truly free.

This replaces the Lenna image with the one from the libav project.

.gitignore

@@ -8,7 +8,7 @@ MANIFEST
 build
 dist
 /tests/media/*
-!/tests/media/lenna.png
+!/tests/media/reference.png
 /venv
 /venv3
 /.env

tests/utils.py

@@ -17,9 +17,10 @@ def get_image_file():
 
     http://en.wikipedia.org/wiki/Lenna
     http://sipi.usc.edu/database/database.php?volume=misc&image=12
-
+    https://lintian.debian.org/tags/license-problem-non-free-img-lenna.html
+    https://github.com/libav/libav/commit/8895bf7b78650c0c21c88cec0484e138ec511a4b
     """
-    path = os.path.join(settings.MEDIA_ROOT, 'lenna.png')
+    path = os.path.join(settings.MEDIA_ROOT, 'reference.png')
     return open(path, 'r+b')