change img srcset

image_sizes.php

/*function adjust_image_sizes_attr( $sizes, $size ) {
   $sizes = '(max-width: 300px) 300px, (max-width: 768px) 768px, (max-width: 1024px) 1024px, (max-width: 1500px) 1500px, 1024px';
   return $sizes;
}
add_filter( 'wp_calculate_image_sizes', 'adjust_image_sizes_attr', 10 , 2 );*/

//* Add custom images instead of default resized from srcset
//add_filter( 'wp_calculate_image_srcset', 'ig_add_custom_image_srcset', 10, 5 );
function ig_add_custom_image_srcset( $sources, $size_array, $image_src, $image_meta, $attachment_id ){
			
	$image_pieces = explode ( '-l.', $image_src);

	if ( !empty($image_pieces) ) {

		$image_src_1500 = $image_src;

		$image_src_900 = $image_pieces[0] . '-m.' . $image_pieces[1];

		$image_src_480 = $image_pieces[0] . '-s.' . $image_pieces[1];

		$width = array('480', '900', '1500');

		//reinstantiate $sources array to empty WP generated srcset
		$sources = array();	

		foreach ($width as $w) {
			$sources[$w] = array(
    	         'url'        => ${'image_src_'.$w},
    	         'descriptor' => 'w',
    	         'value'      => $w,
    	  		);
		}
	
		//var_dump($sources);
	
		//return sources with new srcset value
		return $sources;

	} //end if
}

function ig_resp_images() {
	global $post;
	$css = '';

	if ( have_posts() ) : while ( have_posts() ) : the_post();
 
        $attachments = get_children( array(
            'post_type'   => 'attachment',
	     'post_mime_type' => 'image',
            'numberposts' => -1,
            'post_parent' => 278
        ) );
        print_r($attachments); 
        if ( $attachments ) {
           foreach ( $attachments as $attachment ) {
		
		$image_pieces = explode ( '-l.', wp_get_attachment_url( $attachment->ID ) );
		if ( !empty($image_pieces) ) {
			$css .= '
			.'.$attachment->ID.' { background-image: url();}
			';

		}

           }
        }
	endwhile; endif; 

	if( !empty( $css ) )
		wp_add_inline_style( 'child-theme', $css );
}
//add_action( 'wp_enqueue_scripts', 'ig_resp_images' );