Potresti fare cosi:

nella pagina dove ti richiami le miniature:

codice:
[img]resize_foto.php?w=100&h=100&filename=<?=$file_foto?>[/img]

La pagina resize_foto.php ti crea l'immagine 100X100

codice:
<?php
// The file
$filename = $_GET['filename'];

// Set a maximum height and width
$width = $_GET['w'];
$height = $_GET['h'];

$exst=substr($filename,-3);

// Content type
//header('Content-type: image/jpeg');

// Get new dimensions
list($width_orig, $height_orig) = getimagesize($filename);

if (($width_orig>$width) || ($height_orig>$height)){

	if ($width && ($width_orig < $height_orig)) {
   		$width = ($height / $height_orig) * $width_orig;
	} else {
   		$height = ($width / $width_orig) * $height_orig;
	}

}else{

	$width=$width_orig;
	$height=$height_orig;
	
}
// Resample
	$image_p = imagecreatetruecolor($width, $height);
	if ($exst=="gif"){
		$image = imagecreatefromgif($filename);	
		imagecopyresampled($image_p, $image, 0, 0, 0, 0, $width, $height, $width_orig, $height_orig);
		// Output
		imagegif($image_p);
	}else{
		$image = imagecreatefromjpeg($filename);	
		imagecopyresampled($image_p, $image, 0, 0, 0, 0, $width, $height, $width_orig, $height_orig);
		// Output
		imagejpeg($image_p, null, 100);
	}

	
	
?>
Spero ti possa essere utile.