I installed the plugin and checked the responsiveness of the pictures on my mobile and it worked fine when it was the default pictures. Your default pictures show their dimensions being 1400 x 702.
I resized my background images to match this size. However once I add my own image it no longer is responsive on my phone. The image does not resize to the background on my mobile.
IS there anything else I need to do to get the issue responsive?
https://wordpress.org/plugins/responsive-full-width-background-slider/