Use the post's featured image when the EasyRecipe doesn't have an image.

easyrecipe-convert-with-post-thumbnail.php

<?php
/**
 * Use the post's featured image when the EasyRecipe doesn't have an image.
 *
 * @param integer $image_id Image ID to be used.
 * @param object  $recipe   Tasty Recipe object.
 * @param integer $post_id  Original post ID.
 */
add_filter( 'tasty_recipes_convert_easyrecipe_image_id', function( $image_id, $recipe, $post_id ) {
	if ( $image_id ) {
		return $image_id;
	}
	return get_post_thumbnail_id( $post_id );
}, 10, 3 );