Sanitize image upload in Customizer

customizer-image-sanitization.php

<?php
/**
 * Sanitizes Image Upload.
 *
 * @param string $input potentially dangerous data.
 */
function themepatio_sanitize_image( $input ) {
	$filetype = wp_check_filetype( $input );
	if ( $filetype['ext'] && wp_ext2type( $filetype['ext'] ) === 'image' ) {
		return esc_url( $input );
	}
	return '';
}