uses Jpeg, IdBaseComponent, IdComponent, IdTCPConnection, IdTCPClient, IdHTTP;

{ .... }

function DownloadJPGToBitmap(const URL : string; ABitmap: TBitmap): Boolean;
var
  idHttp: TIdHTTP;
  ImgStream: TMemoryStream;
  JpgImage: TJPEGImage;
begin
  Result := False;
  ImgStream := TMemoryStream.Create;
  try
    idHttp := TIdHTTP.Create(nil);
    try
      idHttp.Get(URL, ImgStream);
    finally
      idHttp.Free;
    end;
    ImgStream.Position := 0;
    JpgImage := TJPEGImage.Create;
    try
      JpgImage.LoadFromStream(ImgStream);
      ABitmap.Assign(JpgImage);
    finally
      Result := True;
      JpgImage.Free;
    end;
  finally
    ImgStream.Free;
  end;
end;


// Example:
// Beispiel:

procedure TForm1.Button1Click(Sender: TObject);
begin
  DownloadJPGToBitmap('http://www.sample.com/test.jpg', Image1.Picture.Bitmap);
end;

（出处：DelphiFans.com）