Flexible Image Example

HTML

<div id="a">
    <div id="b">
        <div id="c"></div>
    </div>
</div>

CSS

#a
{
    width:80%;
    border:1px solid red;
}
#b
{
    width:50%;
    border:1px solid blue;
}
#c {width:300px;height:150px;background:green}
* {text-align:center;margin:auto}