Code:

type
  TMainForm = class(TForm)
 ....
private
    FClientInstance : TFarProc;
    FPrevClientProc : TFarProc;
    procedure ClientWndProc(var Message: TMessage);
end;

implementation

Procedure TMainForm.ClientWndProc(Var Message : TMessage);
var
dc : hdc;
row, col : integer;
begin
with message do
     case msg of
     wm_erasebkgnd:
     begin
     dc := twmerasebkgnd(message).dc;
     for row := 0 to clientheight div wallpaper.picture.height do
       for col := 0 to clientwidth div wallpaper.picture.width do
          bitblt(dc, col * wallpaper.picture.width, row * wallpaper.picture.height,
          wallpaper.picture.width, wallpaper.picture.height,
          wallpaper.picture.bitmap.canvas.handle, 0, 0, srccopy);
       result := 1;
       end;
     else
     Result := Callwindowproc(FPrevclientproc, clienthandle,msg, wparam, lparam);
     end;
end;


procedure TMainForm.FormCreate(Sender: TObject);
begin
  Fclientinstance := makeobjectinstance(clientwndproc);
  FPrevClientProc := Pointer(Getwindowlong(Clienthandle, gwl_wndproc));
  SetWindowLong(Clienthandle, gwl_wndproc, longint(Fclientinstance));
end;