Lots of pictures of all shapes and sizes can be added in one go to this gallery by dragging and dropping them as explained in that yellow note above.
What you see above is a screen-shot of the Site Designer canvas, and a demonstration of the gallery can be seen here. To get back to this page from the gallery, click on the 'Home' button, or use the browser's back button.
The top of the gallery can be edited to hold your own logo, text or buttons. But it should not have too much contents to make it wrap into several lines.
The blue background colour can be changed to match the colour scheme of your site. The colour of the borders and the buttons (if you need them) have to be changed separately.
If you forget to click on the blue background before adding images, you risk that the images go below the blue and into the page body. Just delete the images and try again.