WordPress function to alter the main query to grab posts of the attachment type

sjf_deh_alter_main_query.php

<?php
/**
 * Alter the main query to grab posts of the attachment type
 *
 * @param object $query The WP_Query for the current page view
 */
function sjf_deh_alter_main_query( $query ) {
    
	// we don't want to alter the query in wp-admin
	if ( is_admin() ) { return; }

	// we don't want to alter the query if it's not the main query
   	if ( !$query->is_main_query() ) { return; }
    	
	// furthermore, we only want to do this on the home page
	if( !is_home() ) { return; }
	
   	// I happen to be a big fan of the random, in all things.
	$query->set( 'orderby', 'rand' );

   	// curate our images such that only our best shots make the cover
	$tax_query = array(
		
    		// we've tagged our best photos
		'taxonomy' => 'post_tag',
		
		// the tag we applied to our best photos was "Cover Shot"
		'terms' => 'cover-shot',

		// we grab by slug since slugs seem to get broken or changed less frequently than titles or IDs
		'field' => 'slug',

		// this is the wp default, but I often like to be verbose/redundant so as to help code readability
		'operator'=> 'IN'
	);
	
	// add our tax queries into the array of tax_query queries
	$query->tax_query->queries[] = $tax_query;

	// add the tax_query query to the current query
	$query->query_vars['tax_query'] = $query->tax_query->queries;
 
	// tell wordpress to only grab posts of the attachment post_type
 	$query->set( 'post_type', 'attachment' );

	// because attachments work a little differently than normal posts, we have to tell wordpress to ignore their post_status
	$query->set( 'post_status', 'any' );

	// since we're just doing one large image, we might as well only query 1
	$query->set( 'posts_per_page', '1' );

	// there's no need to return a value because $query is altered by reference

}
add_action( 'pre_get_posts', 'sjf_deh_alter_main_query' );