use Custom Post Template for Image Post Format

add_filter( 'theme_mod_jnews_single_blog_template', 'change_jnews_single_post_template' );
add_filter( 'theme_mod_jnews_single_blog_custom', 'change_jnews_custom_single_post_template' );

/**
 * Change JNews single post tempalte with custom
 *
 * @param string $template Template name.
 * @return string
 */
function change_jnews_single_post_template( $template ) {
    if ( 'image' === get_post_format() ) {
        return 'custom';
    }
    return $template;
}

/**
 * Change JNews custom post template id.
 *
 * @param int|string $template Single Post Template ID.
 * @return integer|string
 */
function change_jnews_custom_single_post_template( $template ) {
    if ( 'image' === get_post_format() ) {
        return 248; /* Change this with your Custom Single Post Template ID  */
    }
    return $template;
}