Displaying Post Attachment(s) at Top of single.php

Attachments are considered children of the post they’re attached to, so this should work:

$images=get_children( array('post_parent'=>$post->ID,
                            'post_mime_type'=>'image',
                             'numberposts'=>1));
echo wp_get_attachment_image($images[0]->ID, 'large');

for a large image… replace “large” with the size definition you want or a width,height array.