WordPress | Set a default featured image (post thumbnail) to each post, based on the term (e.g., Tag) that is assigned to the post. Essentially this sets a featured image for the custom terms you specify (of a specified custom taxonomy).

post-featured-image-based-on-term.php

<?php 

function default_category_featured_image() {
	global $post;
	$featured_image_exists = has_post_thumbnail($post->ID);
	$terms = get_the_terms($post->ID, 'jobtype');
	$number = sizeof ($terms);
	if (!$featured_image_exists)  {
		$attached_image = get_children( "post_parent=$post->ID&post_type=attachment&post_mime_type=image&numberposts=1" );
		if ($attached_image) {
			foreach ($attached_image as $attachment_id => $attachment) {
				set_post_thumbnail($post->ID, $attachment);
			}
		}
		// if multiple terms
		else if ( $number > 1 ) {
			set_post_thumbnail($post->ID, '4650');
		}
		// assistant
		else if ( has_term( 'assistant', 'jobtype' ) ) {
			set_post_thumbnail($post->ID, '2573');
		}
		// chefs
		else if ( has_term( 'chefs', 'jobtype' ) ) {
			set_post_thumbnail($post->ID, '2571');
		}
		// childcare
		else if ( has_term( 'childcare', 'jobtype' ) ) {
			set_post_thumbnail($post->ID, '2572');
		}
		// IF NO TERMS
		else {
			set_post_thumbnail($post->ID, '4650');
			wp_reset_postdata();
		}
	}
}
add_action('the_post', 'default_category_featured_image');