How to control a scroll processing of a GtkScrolledWindow?

[mili <milimeter 163 com>]

Hello, Every body

  I put a GtkImage into a GtkScrolledWindow. If the image is very large, I 
intend to make it scrolling down by press the "PageDown" key. But I don't 
know the correct way to implement this. I tried the following code (in 
Pascal, PageUp case):
>>>>>>
adjust:=gtk_scrolled_window_get_vadjustment(PGtkScrolledWindow(ImageWindow_sw));
with adjust^ do begin
  pinpoint:=value;
  if pinpoint<upper then pinpoint:=value+upper;
  if pinpoint>upper then pinpoint:=upper;
  gtk_adjustment_set_value(adjust,pinpoint);
  gtk_adjustment_value_changed(adjust);
  gtk_adjustment_changed(adjust);
end;
<<<<<<

But unfortunately, the image will become a black board.