<?php
$img = imagecreatefromjpeg($image_uploadee); // Tu créer l'image uploadée dans la variable $img
// ps: change la fonction "imagecreatefromjpeg" par rapport à l'extension

$imgX = imagesx($img);
$imgY = imagesy($img);

$XmaxSizeRequired = 100; // 100 pixels max de longueur !
$YmaxSizeRequired = 100; // 100 pixels max de largeur !

$Xtest = $imgX - $XmaxSizeRequired;
$Ytest = $imgY - $YmaxSizeRequired;

if(($Xtest <= 0)&& ($Ytest <= 0)) // Si l'image est plus petite que 100x100, on ne change rien
{
        $destination = imagecreatetruecolor($imgX,$imgY);
}
else // Sinon on la redimentionne ...
{
        if($imgX < $imgY)
        {       $Ynew = $YmaxSizeRequired;
                $Xnew = ($XmaxSizeRequired * ($imgX / $imgY));
                $destination = imagecreatetruecolor($Xnew,$Ynew);
        }
        elseif($imgX > $imgY)
        {       $Xnew = $XmaxSizeRequired;
                $Ynew = ($YmaxSizeRequired * ($imgY / $imgX));
                $destination = imagecreatetruecolor($Xnew,$Ynew);
        }
        elseif($imgX == $imgY)
        {       $Ynew = $YmaxSizeRequired;
                $Xnew = $XmaxSizeRequired;
                $destination = imagecreatetruecolor($Xnew,$Ynew);
        }
}

$Xdest = imagesx($destination);
$Ydest = imagesy($destination);

// On crée la miniature
imagecopyresampled($destination, $img, 0, 0, 0, 0, $Xdest, $Ydest, $imgX, $imgY);

imagejpeg($destination, 'tiny_avatar.jpg');
?>