This code is taken from: http://www.eighttwentydesign.com/blog/all/149-how-to-get-article-s-intro-image-in-joomla-using-php
They provide a more detailed explanation of how to use this code. Note, I had to make one change from their original code to include a / character after the img src=' text.
<?php
// Article Image
$article = JTable::getInstance("content");
$article->load(JRequest::getInt("id")); // Get Article ID
$article_images = $article->get("images"); // Get image parameters
$pictures = json_decode($article_images); // Split the parameters apart
// Print the image
echo "<img src='/" . $pictures->{'image_intro'} . "' alt='" . $pictures->{'image_intro_alt'} . "'>";
?>