Try these steps:
1. Put your background picture in as tiled background, not sprite.
2. Change it's origin point to 0,0 and then move it to position 0,0.
3. Resize it so that it's y(if you want height) is twice the size of your layout.
4. Add a bullet behavior to it.(speed= whatever you want, acceleration=0, gravity=0, bounce of solids=no, set angle=no, initial state=enabled)
5. Add event: on start of layout>tiled background-set bullet angle of motion to xx degrees(90 degrees if you want it to go down or 270 if you want it to go up)
6. Add another event: tiled background- y is less or equal to Self.Height / 2 action: tiled backround set y to Self.Y + Self.Height / 2