.boundingbox {
width: 400px;
height: 500px;
border: 2px solid #F63;
}
img{
width:400px;
max-height: 500px;
height:auto;
}
I'm editing my answer to further explain my soluton as I've got a down vote.
With the styles set as shown above in css, now the following html div will show the image always fit width wise and will adjust hight aspect ratio to width. Thus image will scale to fit a bounding box as asked in the question.
<div class="boundingbox"><img src="image.jpg"/></div>