Add the featured image to output of Download Details widget
<?php
/**
* Plugin Name: Easy Digital Downloads - Download Details Featured Image
* Author: Easy Digital Downloads
* Author URI: https://easydigitaldownloads.com
* Description: Add the featured image to output of Download Details widget
* Version: 1.0
*/

function sd_edd_download_details_widget_thumbnail( $instance ) {
	
	// get the ID of the current post
	$post_id = get_the_ID();
	
	// grab featured image of the appropriate download
	if ( 'current' == $instance['download_id'] ) {
		echo get_the_post_thumbnail( $post_id );
	} else {
		echo get_the_post_thumbnail( $instance['download_id'] );
	}
}
add_action( 'edd_product_details_widget_before_purchase_button', 'sd_edd_download_details_widget_thumbnail' );
To implement this snippet on your site, create a brand new, empty WordPress plugin and paste the above code inside.