About a year ago I wrote about how to change the desktop wallpaper using SystemParametersInfo(SPI_SETDESKWALLPAPER).

Windows 8 desktop apps (not Store apps) can use the new IDesktopWallpaper API to get a more fine level of control.  So I wrote an app which uses the new API, though I just set the background on all monitors to the same image path, and I don't exercise any of the advanced features of the API.

Pseudocode:

CoInitialize
CoCreateInstance(DesktopWallpaper)
pDesktopWallpaper->SetWallpaper(NULL, full-path-to-image-file)
pDesktopWallpaper->Release()
CoUninitialize

Usage:

>desktopwallpaper.exe "%userprofile%\pictures\theda-bara.bmp"
Setting the desktop wallpaper to C:\Users\MatEer\pictures\theda-bara.bmp succeeded.

Source and binaries attached