If you want display an image that is inserted into your content (a hotlinked image, for example), you must use a function like this (source):
add in functions.php:
function catch_that_image() {
global $post, $posts;
$first_img = '';
ob_start();
ob_end_clean();
$output = preg_match_all('/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', $post->post_content, $matches);
$first_img = $matches [1] [0];
if(empty($first_img)){ //Defines a default image
$first_img = "/images/default.jpg";
}
return $first_img;
}
Then place <?php echo catch_that_image() ?>
where you want display the image.
Note: a hotlinked image just placed in your content can’t be set as Featured Image, a bultin WordPress’feature.