Hello,

Thank you very much for choosing our theme and for contacting us.

This is the Slider Revolution plugin. Please read and follow the suggestions on how to set the background for the slide https://www.themepunch.com/revslider-doc/main-background/

First of all, enter the editing mode of the page where the Slider Revolution displayed. Check which one is used as shown here: http://prntscr.com/isshej

Then find this slider in the Slider Revolution plugin as shown: http://prntscr.com/issi0h

Best Regards