When you press PageDown while the lower side of the image is outside of the window, it will stop scroll down when the image top is in window top, and prevent upper side of image will be outside of the window.
It is similar at PageUp too.
It process images, movies, and flashes.
Additional key assign:
It is a transplant version of Greasemonkey version.
Additional function from Greasemonkey version:
To specify the scroll position, view border line and/or color read (or unread) area.