Problem with CAPTCHA not appearing

[eluser]Unknown[/eluser]
I run a website with software built on CodeIgniter, and I'm having problems with the CAPTCHA image people see when signing up for the site (screenshot attached). The CAPTCHA image is not appearing at all. I get no error message, but when I reload the page, I get the following error message:

A PHP Error was encountered
Severity: Warning
Message: unlink(temp/.jpg): No such file or directory
Filename: controllers/user.php
Line Number: 90

Looking at user.php at line 90, I see the following code (lines 86-91 below):

Code:

// If user failed CAPTCHA, delete it
  if($this->session->flashdata('captcha'))
  {
   // unlink('temp/'.$this->session->flashdata('captcha'));
   unlink(realpath('captcha').'temp/'.$this->session->userdata('time').'.jpg');
  }

What could be the problem here? I checked the permissions on the /temp/ folder in root, and it is writable (777). Please help!