imagesx() and imagesy() error when creating tumbnail

I want to create thumbnail image for top posts automaticly. So I decided to resize current image of post with small sizes for using it as tumbnail using this - resize-class.php class and wrote the code like this:

// Include the class

include("resize-class.php");



$top_posts = mysql_query("In this query I select top posts - the most viewed posts of the day")

while ($row = mysql_fetch_array($top_posts)){

    $post_id    = $row['post_id']; // defining the ID of the post as variable

    $post_image = $row['post_image']; //defining the image of the post as variable

    //save image

    $resizeObj = new resize($post_image); // Initialise - load image

    $resizeObj -> resizeImage(260, 210, 'exact'); // resizing $post_image with 'exact' option

    $resizeObj -> saveImage('images/'.$post_id.'.jpg', 80); // saving thumbnail of $post_image to the storage

}

But returns the errors in loop like this:

Warning: imagesx() expects parameter 1 to be resource, boolean given in resize-class.php on line 34



Warning: imagesy() expects parameter 1 to be resource, boolean given in resize-class.php on line 35



Warning: imagecopyresampled() expects parameter 2 to be resource, boolean given in resize-class.php on line 77

What is the problem ?