In 1941 the National Park Service commissioned noted photographer
Ansel Adams to create a photo mural for the Department of the Interior
Building in Washington, DC. The set of over 200 photos of National
Parks (now in the public domain) is held by the US National Archives
(http://www.archives.gov/research_room/research_topics/ansel_adams_photographs.html)
who provides access to digital copies via
http://www.archives.gov/research_room/arc/) The photos available from
the National Archives web site are unfortunately in a medium
resolution gif format (example:
http://media.nara.gov/media/images/14/14/14-1301a.gif) making them
unsuitable for reproduction, screen saver, or wallpaper use.

My question is where can I download all those Ansel Adams pictures for
free in a higher resolution, non-gif (jpeg is fine) format?

I have found bits and pieces at the following addresses but not the
whole collection:
http://masters-of-photography.com/A/adams/adams.html
http://www.worldvillage.com/wv/cafe/html/reviews/ansel.htm
http://www.archives.gov/exhibit_hall/picturing_the_century/portfolios/port_adams.html
http://www.drowlord.com/art/ansel_adams/thumbnails01.html