On start of layout --> bg --> Set custom movements enabled
This event just tells to the computer that we want to use custom movements.
On start of layout --> bg --> Set custom movement vertical speed to 50
This events sets the vertical speed of background to 50 pixels per second.
On start of layout --> bg --> Accelerate custom movement 50 at angle 270
This event starts to move background downwards at 50 pixels per second. Speed needs to be same as above. If we won't use this, the background will never start to go anywhere.
Before you go to the next event, you need to count how long does it take to that background image to reach its maximum height. If it takes 20 seconds, next event will be:
Every 20.0 seconds --> bg --> Reverse custom movement (both)
Every 25.0 seconds --> bg --> Reverse custom movement (both)
This event will reverse the movement every 25 seconds. This event makes background go back and forth until you exit that layout or tell custom movement to stop.
Now all is ready and you're ready to go! Hope this helped you somehow.
You can download example capx and my background image from left as zip file.