CAPTCHA with images of objects

This CAPTCHA is not text but basically images of objects that the users has to identify between an array of options. My current code is kind-of slow because the query takes almost .7s to execute and it has to run three times.

I'm looking for a way to improve it and make it faster.

// Loop through to display the pictures and options
for ($i = 0; $i < 2; $i ++) {
    $answers = array();

    // Fill an array with 5 random objects unique objects
    $result = mysqli_query($link, "SELECT MAX(captcha_images.image_code) AS image_code, captcha_objects.name FROM captcha_objects LEFT JOIN captcha_images ON captcha_images.object_id = captcha_objects.id WHERE captcha_images.approved = 1 GROUP BY captcha_objects.name ORDER BY RAND() LIMIT 5") or die(mysqli_error($link));
    while($row = mysqli_fetch_assoc($result)) $answers[] = array("code" => $row['image_code'], "name" => $row['name']);

    // Pick the first one as the correct answer
    $_SESSION['captcha_ans'][] = $answers[0];
    $chosen_objects_id[] = mysqli_get_value("id", "captcha_objects", "name", $answers[0]['name'], false);

    // Shuffle 'em
    shuffle($answers);

    // show picture
    $image = ImageCreateFromString(base64_decode($_SESSION['captcha_ans'][$i]['code']));

    // Generate the output of the image, encode it and include it as a data URI
    ob_start();
    imagejpeg($image);
    $base64 = base64_encode(ob_get_contents());
    ob_end_clean();
    echo '<div><img src="data:image/jpeg;base64,'.$base64.'"><br><select name="answer[]"><option value=""></option>';

    // show answers
    foreach ($answers as $answer)
        echo '<option value="'.$answer['name'].'">'.$answer['name'].'</option>';

    // close options
    echo '</select></div>';
    unset($answers);
}