procedure TForm1.Button1Click(Sender: TObject);
var 
  Reg         :TRegistry;
  bmpFile:String;
 
begin

  Reg:=TRegistry.Create;
  Reg.OpenKey('Control Panel\Desktop', false);
  bmpFile:='C:\PICT\MEMORY.BMP';
  Reg.WriteString('WallPaper',bmpFile);

  Reg.CloseKey;
  Reg.Free;

  SystemParametersInfo(SPI_SETDESKWALLPAPER,0,
                                             PChar(bmpFile),
                                             SPIF_UPDATEINIFILE+
                                             SPIF_SENDWININICHANGE);
end;
