See IActiveDesktop::GetWallpaperOptions and IActiveDesktop::SetWallpaperOptions

     int GetWallpaperOptions(ref WALLPAPEROPT pwpo, int dwReserved);

     int SetWallpaperOptions(ref WALLPAPEROPT pwpo, int dwReserved);

    Function GetWallpaperOptions(ByRef pwpo As WALLPAPEROPT, ByVal dwReserved As Integer) As Integer

    Function SetWallpaperOptions(ByRef pwpo As WALLPAPEROPT, ByVal dwReserved As Integer) As Integer

public struct WALLPAPEROPT

     public static readonly int SizeOf = Marshal.SizeOf(typeof(WALLPAPEROPT));

Public Structure WALLPAPEROPT

    WALLPAPEROPT.SizeOf = Marshal.SizeOf(GetType(WALLPAPEROPT))

WALLPAPEROPT opts = new WALLPAPEROPT();

opts.dwSize = WALLPAPEROPT.SizeOf;