CSS: image auto resize and centered inside a div

<div style="width:600px; height:400px;">
<div style="vertical-align:middle; text-align:center; width:100%; height:100%;">
    <img style="max-width:100%; max-height:100%; width:auto; height:100%;" src="http://your-domain.org/your-img.png"/>
</div>
</div>

see also

• CSS force image resize and keep aspect ratio