Kod:
unit Unit1;


interface


uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;


type
  TForm1 = class(TForm)
    Button1: TButton;
    Label1: TLabel;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;


var
  Form1: TForm1;


implementation


{$R *.dfm}


procedure TForm1.Button1Click(Sender: TObject);
var
  s: string;
begin
  s := 'c:\windows\Greenstone.bmp';
  SystemParametersInfo(SPI_SETDESKWALLPAPER, 0, PChar(s),0);
end;


end.