To have random access, all the images have to be preloaded because you don't know which one the reader will choose to view next. With sequential viewing, the script knows which is the next image to preload in the background.
If you don't preload the images, there will be a noticable delay between clicking on the thumbnail and the image appearing, but even that is preferable to having to wait for them all to load before you can view the first one.
If you don't preload the images, there will be a noticable delay between clicking on the thumbnail and the image appearing, but even that is preferable to having to wait for them all to load before you can view the first one.