Help me find what is wrong with this script

Hi, I didn't write this and was holding off on fixing it as I wanted to get the whole application overhauled but finding a coder has been difficult so I need to try to fix at least this part myself. It is a captcha script that creates an image of a math problem and the visitor must give the correct answer. However, more often that not even the correct answer is not accepted. I think there may be a sessions issue as well, because when you leave a the page and go back to it - the math problem doesn't update.

Here is the verify class

<?php

class Verify {

	var $verify;
	var $session_id;
	
	function Verify() {
	
		$this->session_id = session_id();
		
	}
	function get_image_verify() {
	
		/* SEND HEADER */
		header ("Content-Type: image/jpg");

		$new_string;

		// Set up the image holder
		$im = ImageCreate(200, 40);  

		// Background=black fore = white
		$white = ImageColorAllocate($im, 255, 255, 255);
		$black = ImageColorAllocate($im, 0, 0, 0);

		/* CODE MODIFICATION ON : 22/08/2008 */
		/* CHANGE FROM A FULL RANDOM STRING, TO A MATHS PUZZEL */
		
			/*random string generator.*/
			/*The seed for the random number*/
			//srand( ( double ) microtime() * 1000000 );
			
		// Generate 2 random numbers between 1 and 100
		$number1 = rand ( 51, 100 );
		$number2 = rand ( 1, 50 );
		
		// Get random number between 1 and 2
		// 1 = PLUS 2 = MINUS
		$plusminus = rand ( 1, 2 );
		
		// Create string
			//$string = md5(rand (0,9999) );
			$string = ( $plusminus == 1 ) ? "$number1 + $number2 =" : "$number1 - $number2 =";

			// Create new string
			//$new_string = substr( $string, 17, 5 );

		// Set session var to hold the string value	
			//$_SESSION['inped_image_string'] = $new_string;
		$_SESSION['inped_image_string'] = ( $plusminus == 1 ) ? $number1 + $number2 : $number1 - $number2;

		// Fill the background
		ImageFill( $im, 0, 0, $black );

		// WRITE THE STRING AT COORDS (70,10) IN WHITE.
			//ImageString( $im, 4, 70, 10, $new_string, $white );
		ImageString ( $im, 4, 70, 10, $string, $white );

		// Write images file
		$this->verify = "./images/verify/" . $this->session_id . ".png";
		
		ImagePNG ( $im, $this->verify );
		
		ImageDestroy( $im );
		
		// Revert header
		header ( "Content-type: text/html" );
		

	}
	
	function get_image_string() {
	
		return ( $_SESSION['inped_image_string'] );
		
	}
	
	function clear_cache() {
	
		$time_now = strtotime ( date ( "H:i:s", time() ) );
		$folder = "./images/verify/";
		
		$of = opendir ( $folder );
		
		while ( $f = readdir ( $of ) ) {

			if ( $f != "." && $f != ".." ) {
			
				$file_time = filectime ( $folder . $f );
				
				$diff = $file_time - $time_now;
				
				if ( $diff > -3600 ) {
				
					unlink ( $folder . $f );
					
					//echo "<b>Time Older Than 1 Hour</b>";
				}
				//echo "File: $f - Modify Time: $file_time - Now Time: $time_now - Time Diff: $diff";
				
			}
		}
	}
}

?>

PHP:

And here is the use of the class with a form

<?php

class Contact {

	var $template;
	var $display;
	
	var $verify;
	
	function Contact() {
	
		// We need the contact form template to be loaded
		$open_file = fopen ( "./templates/contact_form.tpl", "r" );
		$this->template = fread ( $open_file, filesize ( "./templates/contact_form.tpl" ) );

		// Create verfiy object instance
		$this->verify =& new Verify();
		
		$this->verify->clear_cache();
		
		
	}
	
	function replace_tags() {
	
		$this->template = eregi_replace ( "%%verify_image%%", "<img src='" . $this->verify->verify . "' />", $this->template );
		
	}
	
	function contact_form( $extra_content = "" ) {
	
		$functions = new Functions;

		// Setup image verify
		$this->verify->get_image_verify();
		
		// Replace tags in the contact form
		$this->replace_tags();
		
		$form = "<p class='gallery_header' align='center'>Contact Us</p>";
 
		$form .= ( !empty ( $extra_content ) ) ? "<p class='error' align='center'>$extra_content</p>" : "";
		
		$form .= $this->template;
		
		return ( $form );
		
	}
	
	function send() {
	
		if ( $this->verify->get_image_string() == $_POST['verify'] ) {
		
			$to = $this->get_admin_email();
			$from = $_POST['email'];
			$name = $_POST['realname'];
			$message = $_POST['mesg'];
			$subject = "Message From: $name";
			$headers = "Content-type: text/html\n";
			$headers .= "From: " . $from;
			
			mail ( $to, $subject, $message, $headers );
		
			return ( $this->contact_form ( "Your Message Has Been Sent" ) );
			
		} else {
		
			return ( $this->contact_form ( "The Answer You Entered To The Maths Problem Is Incorrect" ) );
			
		}
		
	}
	
	function get_admin_email() {
	
		return ( $this->display->get_kennel_detail ( "Email" ) );
		
	}
	
}

?>

PHP:

Could someone point out where the issue lies? I greatly appreciate any help you can give.