Right now, media is using two different formats for bitmap
images: GIF and PNG. Let's use just one, to make it simpler when
building with Sphinx.

As PNG is usually better than GIF, let's use it.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>