- krsort: documentation ( source)
<?php
$aImages = array (
'1___3tones123.jpg' => '3tones123.jpg',
'10___20002020-BLUE.jpg' => '20002020-BLUE.jpg',
'20___bigeyes01.jpg' => 'bigeyes01.jpg',
'1___bigeyes02.jpg' => 'bigeyes02.jpg',
);
krsort($aImages, SORT_NATURAL);