uses
  RichEdit;

// Stream Callback function
type
  TEditStreamCallBack = function(dwCookie: Longint; pbBuff: PByte;
    cb: Longint; var pcb: Longint): DWORD;
  stdcall;

  TEditStream = record
    dwCookie: Longint;
    dwError: Longint;
    pfnCallback: TEditStreamCallBack;
  end;

// RichEdit Type
type
  TMyRichEdit = TRxRichEdit;

// EditStreamInCallback callback function
function EditStreamInCallback(dwCookie: Longint; pbBuff: PByte;
  cb: Longint; var pcb: Longint): DWORD; stdcall;
  // by P. Below
var
  theStream: TStream;
  dataAvail: LongInt;
begin
  theStream := TStream(dwCookie);
  with theStream do
  begin
    dataAvail := Size - Position;
    Result := 0;
    if dataAvail <= cb then
    begin
      pcb := read(pbBuff^, dataAvail);
      if pcb <> dataAvail then
        Result := UINT(E_FAIL);
    end
    else
    begin
      pcb := read(pbBuff^, cb);
      if pcb <> cb then
        Result := UINT(E_FAIL);
    end;
  end;
end;

// Insert Stream into RichEdit
procedure PutRTFSelection(RichEdit: TMyRichEdit; SourceStream: TStream);
  // by P. Below
var
  EditStream: TEditStream;
begin
  with EditStream do
  begin
    dwCookie := Longint(SourceStream);
    dwError := 0;
    pfnCallback := EditStreamInCallBack;
  end;
  RichEdit.Perform(EM_STREAMIN, SF_RTF or SFF_SELECTION, Longint(@EditStream));
end;

// Convert Bitmap to RTF Code
function BitmapToRTF(pict: TBitmap): string;
// by D3k
var
  bi, bb, rtf: string;
  bis, bbs: Cardinal;
  achar: ShortString;
  hexpict: string;
  I: Integer;
begin
  GetDIBSizes(pict.Handle, bis, bbs);
  SetLength(bi, bis);
  SetLength(bb, bbs);
  GetDIB(pict.Handle, pict.Palette, PChar(bi)^, PChar(bb)^);
  rtf := '{\rtf1 {\pict\dibitmap ';
  SetLength(hexpict, (Length(bb) + Length(bi)) * 2);
  I := 2;
  for bis := 1 to Length(bi) do
  begin
    achar := Format('%x', [Integer(bi[bis])]);
    if Length(achar) = 1 then
      achar := '0' + achar;
    hexpict[I - 1] := achar[1];
    hexpict[I] := achar[2];
    Inc(I, 2);
  end;
  for bbs := 1 to Length(bb) do
  begin
    achar := Format('%x', [Integer(bb[bbs])]);
    if Length(achar) = 1 then
      achar := '0' + achar;
    hexpict[I - 1] := achar[1];
    hexpict[I] := achar[2];
    Inc(I, 2);
  end;
  rtf := rtf + hexpict + ' }}';
  Result := rtf;
end;


// Example to insert image from Image1 into RxRichEdit1
procedure TForm1.Button1Click(Sender: TObject);
var
  SS: TStringStream;
  BMP: TBitmap;
begin
  BMP := TBitmap.Create;
  BMP := Image1.Picture.Bitmap;
  SS := TStringStream.Create(BitmapToRTF(BMP));
  try
    PutRTFSelection(RxRichEdit1, SS);
  finally
    SS.Free;
  end;
end;