I had trouble making my full-width slider work on my child theme. I would upload background images with the correct width, but the theme cropped them and tiled to fill the slider width.
Syahir explains how to add an image width filter in your child theme’s functions.php file in the original discussion thread:
add_filter( 'graphene_slider_full_image_width', 'graphene_custom_slider_image_size' );
That still didn’t work for me. I decided he must have meant the snippet to instead read:
add_filter( 'graphene_slider_image_width', 'graphene_custom_slider_image_size' );
Removing the word “_full” in the first parameter is the only difference.