Thinking more...

static gboolean expose_event( GtkWidget* widget, GdkEventExpose* event, gpointer data )
{
	GError* error=NULL;
	int width=widget->allocation.width, height=widget->allocation.height;

	GdkPixbuf* buf=gdk_pixbuf_new_from_file_at_scale( "your_photo.jpg", &error );
	if( buf==NULL )
		g_print("load fail.\n" );
	else
	{
		bufWidth = gdk_pixbuf_get_width( buf );
		bufHeight = gdk_pixbuf_get_height( buf );
		gdk_draw_pixbuf( widget->window, NULL, buf, 0, 0, 0, 0, 
				(width>bufWidth?bufWidth:width), (height>bufHeight?bufHeight:height), 
				GDK_RGB_DITHER_NORMAL, 0, 0 );
		g_object_unref( buf );
	}
}

int main( int argc, char* argv[])
{
	GtkWidget* drawing_area=NULL;

	// 省略一萬行
	g_signal_connect( G_OBJECT(drawing_area), "expose_event", G_CALLBACK( expose_event ), NULL );

	// 再省略兩萬行...	
}