How do I get the image that is submitted with a post, when I’m in the loop?

You can use this to identify the attachments in a post:

$args = array(
    'post_type' => 'attachment',
    'post_parent' => $post->ID
);
$images = get_posts($args);
foreach($images as $image){
   echo wp_get_attachment_image($image->ID, 'size')
}