Just go into the website's "Settings" dialog (from the drop-down menu in the upper, right-hand corner of Website Maker) and adjust the height there. That will enable you to extend the website's vertical dimension. Keep in mind that there's a limit, though. This function is just designed to make use of the full screen, even at very high resolutions. It's not designed to create websites with endless vertical scrolling bars. You might want to spread out your photos over several pages and possibly create some little sub navigation.