<div id="container">
<div id="thumbnail"><img src="https://www.google.co.uk/images/srpr/logo11w.png" /></div>
<div id="heading">Tilden Regional Park</div>
</div>
The only required styles are:
#thumbnail {
height: 500px;
width: 500px;
margin: 0 auto;
}
#thumbnail img {
height: 100%;
width: 100%;
}
You need to add auto
margins to the element you want centred, this will be #thumbnail
not the image itself, because the image has 100% width and height.