Оформил: DeeCo
Автор: http://www.swissdelphicenter.ch

type
   TForm1 = class(TForm)
     Button1: TButton;
     OpenDialog1: TOpenDialog;
     procedure Button1Click(Sender: TObject);
     procedure FormDestroy(Sender: TObject);
   private
     FWallpaper: TBitmap;
     procedure WMEraseBkGnd(var Msg: TWMEraseBkGnd); message WM_ERASEBKGND;
   end;

 var
   Form1: TForm1;

 implementation

 {$R *.DFM}
 procedure TForm1.FormDestroy(Sender: TObject);
 begin
   FWallpaper.Free;
 end;

 procedure TForm1.Button1Click(Sender: TObject);
 begin
   // Load the bitmap 
  // Bild laden 
  if OpenDialog1.Execute then
   begin
     if not Assigned(FWallpaper) then
       FWallpaper := TBitmap.Create;
     FWallpaper.LoadFromFile(OpenDialog1.FileName);
     Invalidate;
   end;
 end;

 procedure TForm1.WMEraseBkGnd(var Msg: TWMEraseBkGnd);
 var
   row, col: Integer;
 begin
   if not Assigned(FWallpaper) then
     inherited
   else
   begin
     // Draw the bitmap 
    // Das Bild zeichnen 
    for Row := 0 to ClientHeight div FWallpaper.Height do
       for Col := 0 to ClientWidth div FWallpaper.Width do
         BitBlt(Msg.Dc,
           Col * FWallpaper.Width,
           Row * FWallpaper.Height,
           FWallpaper.Width,
           FWallpaper.Height,
           FWallpaper.Canvas.Handle,
           0,
           0,
           SRCCOPY);
     Msg.Result := 1;
   end; { else }
 end;

 end.