In this one I wanted to have the moon rising behind the tree, so I inserted a "background" rather than a positioned scroll because scrolls only move horizontally or vertically, but a background can scroll at a diagonal.  To make it easy on myself, I created the transparent gif for the moon, making it 420x320 pixels (just a little larger than the main graphic so that each completion of the scroll would let the moon completely leave the picture before starting a new display).   I put the moon in the bottom left hand corner of the graphic.  Then I set the background to scroll up and to the right, leaving all other settings alone EXCEPT making sure to check the box that says "Layer with other Objects".  If you don't do this, you won't see the moon.

The effect in this one of having the moon and birds "behind" the tree is accomplished by layering a transparent gif of just the tree over the top of everything else.