code: public function setDefaultImage($image)
  	{
  		// Quick check against boolean false.
  		if($image === false)
  		{
  			$this->default_image = false;
  
  			return $this;
  		}
  
  		// Wipe out the param cache.
  		$this->param_cache = NULL;
  
  		// Check $image against recognized gravatar "defaults", and if it doesn't match any of those we need to see if it is a valid URL.
  		$_image = strtolower($image);
  		$valid_defaults = array('404' => 1, 'mm' => 1, 'identicon' => 1, 'monsterid' => 1, 'wavatar' => 1, 'retro' => 1);
  		if(!isset($valid_defaults[$_image]))
  		{
  			if(!filter_var($image, FILTER_VALIDATE_URL))
  			{
  				throw new InvalidArgumentException('The default image specified is not a recognized gravatar "default" and is not a valid URL');
  			}
  			else
  			{
  				$this->default_image = rawurlencode($image);
  			}
  		}
  		else
  		{
  			$this->default_image = $_image;
  		}
  
  		return $this;
  	}
docs: Set the default image to use for avatars.
  @param mixed $image - The default image to use. Use boolean false for the gravatar default, a string containing a valid image URL, or a string specifying a recognized gravatar "default".
  @return \thomaswelton\GravatarLib\Gravatar - Provides a fluent interface.
  
  @throws \InvalidArgumentException