Prestashop images upgrade between < 1.4 (not 1.4) and 1.5, creates the folders according to the pictures names.

upgradePrestashopPictures.php

<?php
/**
* @author Antoine Bluchet
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program.  If not, see <http://www.gnu.org/licenses/>
*
* Transforms old pictures id-img_id-mode.jpg to /i/m/g/i/d/mode.jpg
* for example 101-415-default.jpg to /4/1/5/default.jpg
*
* Usage : replace $path and launch the script
**/

$path = './img/p'; //Path to your pictures
$copy = true; //Copy, set to false to move the pictures /!\ At your own risk /!\

//Directory separator
define('DS', '/');
//no time limit
set_time_limit(0);

if ($handle = opendir($path)) {

    while (false !== ($entry = readdir($handle))) {
    	if($entry !== '.' && $entry != '..') {
	    	//Explode picture name
	    	$array = explode('-', $entry);

	    	//Unset the first ID
	    	unset($array[0]);
	    	
	    	if(isset($array[1])) {
	    		//Get the numbers which are the directories
	    		if(preg_match_all("/\d+/", $array[1], $matches))
	    			$nb = str_split($matches[0][0]);

			   	$dir = $path . DS . implode(DS, $nb);

			   	//Create dirs recursivly
	    		if(!file_exists($dir)) 
	    			mkdir($dir, 0755, true);

	    		//Copy file path
		    	$copy = $dir . DS . implode('-', $array);

		    	if(!file_exists($copy))
		    		if($copy === true)
				    	copy($path . DS . $entry, $copy);
				    else
				    	rename($path . DS . $entry, $copy);
		    }
	    }
    }
}