画像の真ん中らへんを切り抜いて表示（naturalHeightとnaturalWidthが動作するブラウザのみ）

gistfile1.js

// javascript

var url = "http://jsdo.it/img/common/side/bnr_html5_01.png";

var image = document.getElementsByTagName('img')[0];
image.src = url;
image.onload = function() { 

    // for safari, firefox
    // その他のブラウザは別途対応お願いします
    var height = this.naturalHeight;
    var width  = this.naturalWidth;

    if (height > width) {       

        height *= 200 / width;  
        this.width = 200;
        this.style.top  = - (height - 200) / 2 + 'px';
        this.style.left = 0;
        this.style.position = 'absolute';

    } else {

        width *= 200 / height;  
        this.height = 200;
        this.style.top  = 0;
        this.style.left = - (width - 200) / 2 + 'px';
        this.style.position = 'absolute';
                        
    }                   
};

// html

<div class="photo">
    <img src="" />
</div>


//css

body { background-color: #DDDDDD; font: 30px sans-serif; }

div.photo {
    float: left;
    margin: 5px;
    width: 130px;
    height: 130px;
    position: relative;
    overflow: hidden;
}

    div.photo img {
        display:block;
    }