This is just an example to show how fast gallery page can be loaded, even if it has hundreds of photos. Note that image number is limited initially, while all the other images are accessible in popup mode.
Due to licencing rules and technical limitations, demo images are not included in galleries. You just need to edit this page and add some of your own images to the existing gallery. While on the page edit screen, make sure the content editor is set to visual mode and just click the gallery icon/button.