Down scroll picture for keynote

You could use the Magic Move transition. Arrange the image on multiple sequential slides and you can achieve a scrolling panning effect that pauses at your designated points.

If running the presentation on a Mac you can pause a slide during playing ... Press F key. To resume Press K key again. (I use F means Freeze to remember this one).

This works when text or an image is scrolling using Move In action.

An effective way to show oversized images is to place them into Keynote with some of the image overhanging the edge of the slide then assign a Move action to the image so that it moves the overhanging bit into the slide. This can be combined with re-sizing by scale with interesting effects. Be sure to add some time to the action so that is moves smoothly and gracefully.

Good luck

Is there a way to pause and resume the scroll on the fly? Say if you need to explain things at designated points

But what if I need to be able to pause on the fly, and not at pre-designated points? Thanks so much!

You're amazing! Thank you so much this! =)