Weird result

Hello,

I followed all steps as explained here.

So far so good. But when I put the php code in the page.php of my ‘Twenty Fourteen’ theme I get a http 500 error. And when I put the php code in the single.php of my theme I get a strange result :
the first picture appears as a featured image on top of the content of my post (which is normal) but the other pictures appears as thumbnails in the footer (which is weird).

How can I make this right so that all pictures appear as featured images on top of the text content of my post or page?

Thanx!