Enable job thumbnails and set first attached image as the thumbnail on submission

gistfile1.php

<?php 

// Add post type image support
add_action( 'init', 'enable_job_thumbnails', 50 );

function enable_job_thumbnails() {
	add_post_type_support( 'job_listing', 'thumbnail' );
}

// Set first uploaded attached image as thumbnail
add_action( 'job_manager_update_job_data', 'job_manager_update_job_data_set_thumbnail' );

function job_manager_update_job_data_set_thumbnail( $job_id ) {
	$images = get_posts( 'fields=ids&post_type=attachment&post_mime_type=image&post_parent=' . $job_id );
	if ( $images ) {
		set_post_thumbnail( $job_id, $images[0] );
	}
}