How to change my images device specifically?
To change the Background image of your slides device specifically, you need the Pro version of Smart Slider.
You should select the Slide element -> go to its Style tab -> press the little device icon next to Slide Background.
You can hide layers device specifically from their '...' menus.
So if you want a different image in place of the current one, just put both layers into the same placement, and hide them on different devices.
Slider background image
The slider background image cannot be changed device specifically. If you really want to replace it, you could duplicate your slider and use Hide on, to display different sliders on different devices.