Question:
How do I display a bitmap to the client area of an MDI parent form? 
Answer:
Here are the necessary steps to add wallpaper to a MDI parent
form:


Create a new project
Set the form's FormStyle to fsMDIForm
Drop an image on the form and select a bitmap into it.
Find the { Private Declarations } comment in the form's
   definition and add these lines right after it:

    FClientInstance : TFarProc;
    FPrevClientProc : TFarProc;
    procedure ClientWndProc(var Message: TMessage);

Find the  "implementation"  line and the {$R *.DFM} line that
   follows it. After that line, enter this code:

procedure TMainForm.ClientWndProc(var Message: TMessage);
var
  Dc : hDC;
  Row : Integer;
  Col : Integer;
begin
  with Message do
    case Msg of
      WM_ERASEBKGND:
      begin
        Dc := TWMEraseBkGnd(Message).Dc;
        for Row :=  0  to ClientHeight div Image1.Picture.Height do
          for Col :=  0  to ClientWidth div Image1.Picture.Width do
            BitBlt(Dc,
               Col * Image1.Picture.Width,
               Row * Image1.Picture.Height,
               Image1.Picture.Width,
               Image1.Picture.Height,
               Image1.Picture.Bitmap.Canvas.Handle,
                0 ,
                0 ,
               SRCCOPY);
          Result :=  1 ;
      end;
      else
        Result := CallWindowProc(FPrevClientProc,
                                 ClientHandle,
                                 Msg,
                                 wParam,
                                 lParam);
  end;
end;

In the OnCreate method for the form, type the following lines
of code:

   FClientInstance := MakeObjectInstance(ClientWndProc);
   FPrevClientProc := Pointer(GetWindowLong(ClientHandle,
                              GWL_WNDPROC));
   SetWindowLong(ClientHandle,
                 GWL_WNDPROC, LongInt(FClientInstance));

Add a new form to your project and set its FormStyle property to
fsMDIChild.



Now you have a working MDI project with  "wallpaper"  where the image
bitmap is tiled to cover the MDI form's client area.