The standard way to obtain a wallpapaper is via right-click menu through “Save image as” functionality.
If RunIT-QT or any other minimal browser is being used it is possible to “Copy image address” via right-click menu and then throw it inside a terminal client along with wget in order to download.
Wget example example: wget 123.png
PostX Gnu/Linux 0.4.4.-0.5 wallpapers
PostX Gnu/Linux 0.7 wallpapers