Hide the slide background image on mobile
Smart Slider 3 does not have an option to disable the slide background image on different devices. But there's a trick you can try.
You'd need to set a device specific background for mobile, to hide the "original" desktop image. You can find the option in Expert mode at the Slide settings > Background tab > Image > Image > Manage.
What you could do with this option is that you could create a 1x1px transparent image, for example, at: http://www.1x1px.me and set that for mobile devices. It would replace the original background image, and since it's a different image, the desktop background wouldn't be visible.