How to slide a background picture in Storyline 360 ?

Hello community,

I have a background picture that I have to slide on left and right with the mouse. Picture is 1920*720 and my projet is 1280*720. So I have 320 pixels on each side and I 've create 2 motions paths of 320 pixels (right and left).

I've tried many things :
- A very very big slider that control 2 motion path but not so easy to slide coupled with a variable to control the apparition of blank part on the side. (maybe not the good way).
- A trigger who, when the user click on the 2 arrows on his keyboard, that command motion path (but not appropriate if the user use a mobile device)

And when I mixed the 2 methods, sometimes the user can move the picture more than I would and a blank part appears. Then the picture cannot be replaced at the correct place. 

In fact, it's working but it's not so easy and can have a randomizing behavior.

Do you have any idea? Thx for helping.