Hi resolution images
Gigapixel photographs are single large images of many gigapixels created by stitching together lots of individual photos (typically taken with a long zoom lens) to capture lots of close shots to assemble into one large gigapixel image.
At the moment we've only experimented with 112 mega-pixel images using just 18 photographs to produce a 360 panorama of 15,000 pixels by 7500 pixels high. These only take a few minutes to stitch together, whereas some professional gigapixel images may take hundreds of close-up photographs of individual details of a scene using computer controlled cameras. These setups would make fractional moves between each overlapping picture and then consume lots of computing power and memory to 'stitch' together.
A muti-resolution cube format has been used in order to render the pixels in your browser dynamically (only the parts of the picture you are looking at). If we had used a single large picture to store all the information, the file would take a while to load into your browser window, and on mobile devices would likely exceed the maximum_texture_size of the browser's texture map.