Problems with Draw Loop using u8glib
You haven't shown us all the code – in particular, the call to Draw() from loop() is missing – so this answer is speculative, based on the assumption your loop() is as follows. 

void loop(void) {
    Draw();
}


If so, your first bit of code will constantly redraw because loop() runs repeatedly without delay.  Try adding a delay(1000); after Draw() in that case.

Your second bit of code will begin by drawing your image once, and in that Draw() call it will set needsUpdate to false.  Then loop() runs again, and the firstPage() call erases the screen.  Since needsUpdate now is false and remains false, subsequent calls don't draw an image.

Rather than putting your picture loop into Draw(), perhaps use Draw() just to draw primitives, and keep your controls in loop(): 

void loop(void) {
  Display.firstPage();  
  do {     // picture loop
    Draw();
  } while( Display.nextPage() );    
  delay(1000); // delay before next picture
}


If you still get repeated fast redraws, for troubleshooting comment out the do and } while lines.