Ah ok, non lo avevo postato perchè pensavo che l'errore fosse nel modo in cui includevo e richiamavo la classe   
 
"resizeImage.php" :
	Codice PHP:
	
class resizeImage{
    
function resize($old_image, $new_image, $new_width, $new_height, $quality){
          
            
         ogni variabile 
        
         
         list($old_width, $old_height, $_ext) = getimagesize($old_image);
         
         $image = NULL;
         
         switch($_ext){
             case 1: $image = imagecreatefromgif($old_image);
             break;
             
             case 2: $image = imagecreatefromjpeg($old_image);
             break;
             
             case 3: $image = imagecreatefrompng($old_image);
             break;
             
             case 8: $image = imagecreatefromwbmp($old_image);
             break;
         }
         if($image==NULL){ return false; }
         
         
         $new_size = imagecreatetruecolor($new_width, $new_height);
         if(! imagecopyresampled($new_size, $image, 0, 0, 0, 0, $new_width, $new_height, $old_width, $old_height) ){ return false; }
         
         nome file descritti in "$new_image" e con qualià = $quality
         switch ($_ext) {
                 case 1: imagegif($new_size, $new_image, $quality);
                 break;
                 
                 case 2: imagejpeg($new_size, $new_image, $quality);
                 break;
                 
                 case 3: imagepng($new_size, $new_image, $quality);
                 break;
                 
                 case 8: imagewbmp($new_size, $new_image, $quality);
                 break;
         }
         
         imagedestroy($image);
         imagedestroy($new_size);
    
    return true;    
       
}//END FUNCTION
function resizePerc($old_image, $new_image, $perc, $quality){
    
    list($old_width, $old_height) = getimagesize($old_image);
    
    $new_width = $old_width*$perc/100;
    $new_height = $old_height*$perc/100;
    
    resize($old_image, $new_image, $new_width, $new_height, $quality);
    
}//END FUNCTION
}//END of CLASS 
 
"pagina.php" :
	Codice PHP:
	
include("resizeImage.php");
$res = new resizeImage(); 
 
boh, mi sembra di non aver fatto errori, mentre sicuramente l'ho fatto ma non lo VEDO.