Earlier i developed an application which will download the 2008 and 2009 photo contest wallpapers from national geographic website. I just modified the application which will now download the 2007,2008 and 2009 your shot section wallpapers too. So if you download all together it will be 1382 wallpapers (with out duplicates) with a total size of 245MB.
It will not download if a file with the same name exists on that folder
This is an open source application developed using c# in .Net framework 2.0 and it do not need to install too. You will need to have .net framework2.0 installed.