If you modified the slider width, you would need to modify the width that WordPress automatically crops images into.
You will need a child theme for this. Insert this code in your child theme’s functions.php file (create one if it doesn’t exist yet):
return 445; // Change this value to your slider's width
add_filter( 'graphene_slider_small_image_width', 'graphene_custom_slider_image_width' );
445px is the theme’s default slider width. Change it to your customised width.