adventure child functions.php

<?php
/* Child Theme function by Digital Raindrops */
function raindrops_get_category_image() {
    if ( is_category() ) {
        //Get the active category id
        global $cat;
        if( $cat ) {
            //Find the ID of the first post in the category
            $args = array( 'numberposts' => -1, 'category' => $cat, 'order'=> 'ASC', 'post_status' => 'publish' );
            $myposts = get_posts( $args );
            foreach( $myposts as $catpost ) {
                if ( has_post_thumbnail( $catpost->ID ) ) {
                    $url = wp_get_attachment_image_src( get_post_thumbnail_id( $catpost->ID ), 'single-post-thumbnail' );
                    $path = $url[0];
                    return  esc_url_raw( $url[0] );
                }
            }
            return '';
        }
    }
    return '';
}