Neither.  It downloads the first image and displays it.  While the first image is displayed it downloads the other images in the background.  If you choose an image or pano that has not been loaded yet, it will grab it and display it while still downloading the unseen images.  This is only my observation and I do not have access to the logic behind TW3.  This was also the case with TW1.3 and probably the best approach for maximum usability.