Please add an option to download the HD image.
Its url is HPImageArchive.aspx?format=xml&idx=0&n=1:<urlBase>_1920x1200.jpg, for example http://www.bing.com/az/hprichbg?p=rb%2fRedwoodRays_ROW11815220129_1920x1200.jpg
I believe this picture was introduced with the Bing Desktop application on April 4.
(The regional images (specified by the mkt=en-US etc. argument for HPImageArchive.aspx) are still only available in 958x512. Bing Desktop access HPImageArchive.aspx with the argument mkt=en-ww that as of now return the same data as an absent mkt argument.)