PHP: any way to secure a session variable / captcha?

I've made some image validation. The user has to enter the code from the dynamically generated image.

This code is being used in the main php file:

@session_start();
if (isset($_POST['session_pw'])&&$_SESSION['result']==$_POST['security_im']) {
    $_SESSION['pw']=$_POST['session_pw'];
    $_SESSION['ip'] = $_SERVER['REMOTE_ADDR'];
}
if ($_SESSION['pw']=='mypassword'&&$_SESSION['ip'] == $_SERVER['REMOTE_ADDR']){

        // OK we are in.
    // Otherwise display a login form:
}else echo <<<EOD
    <form method="POST">
    <font color="blue"> Authorization is required:</font><br>
    <input type="text" name="security_im" style="text-align:right;padding-right:1px;background-repeat:no-repeat;background-image:url(../imagecheck.php)"><br>
    <input type="password" name="session_pw" style="text-align:right;padding-right:1px"><br>
    <input type="submit" style="margin:0px">
    </form>
EOD;

Note this:

background-image:url(../imagecheck.php)

This is how I call the generated image. And this is it's content:

<?php
session_start();
create_image();
exit();

function create_image() {
    $md5 = "";
    while (strlen($md5) == 0) {
        $md5 = md5(mt_rand());
        $md5 = preg_replace('/[^0-9]/', '', $md5);
    }
    $pass = substr($md5, 10, 5);
    $_SESSION['result'] = $pass;
    $width = 60;
    $height = 20;
    $image = ImageCreate($width, $height);

    //We are making three colors, white, black and gray
    $clr[1] = ImageColorAllocate($image, 204, 0, 204);
    $clr[2] = ImageColorAllocate($image, 0, 204, 204);
    $clr[3] = ImageColorAllocate($image, 204, 204, 0);

    $R = rand(1, 3);
    $black = ImageColorAllocate($image, 255, 255, 255);
    $grey = ImageColorAllocate($image, 204, 204, 204);

    //Make the background black
    ImageFill($image, 0, 0, $black);

    //Add randomly generated string in white to the image
    ImageString($image, 5, 7, 2, $pass, $clr[$R]);

    //Throw in some lines to make it a little bit harder for any bots to break
    imageline($image, 0, 0, $width - 2, $height - 2, $grey);
    imageline($image, 0, $height - 2, $width - 2, 0, $grey);
    ImageRectangle($image, 0, 0, $width - 1, $height - 1, $grey);

    //Tell the browser what kind of file is come in
    header("Content-Type: image/jpeg");
    //Output the newly created image in jpeg format
    ImageJpeg($image, null, 80);
    //Free up resources
    ImageDestroy($image);
}
?>

Note:

$_SESSION['result'] = $pass;

This is how I store key in the session.

Now, the problem is, the image can be potentially blocked, and the session key will not change from the last stored result. This means a huge threat. I wonder if there any protection or workaround possible?

Please answer only if you understand the question and security :-)