fit.js

// shorter vars for brevity
var a = image.naturalWidth,
    b = image.naturalHeight,
    c = maxImageWidth,
    d = maxImageHeight,
    r1 = a / b, // image aspect ratio
    r2 = c / d; // container aspect ratio

// determine which dimension is the limiting factor
// according to the aspect ratio comparison and 
// calculate the scale ratio
var s = (r1 > r2) ? a / c : b / d;

// calculated the scaled dimensions
var w = a / s;
var h = b / s;

// position the image
imageStyle.marginLeft = -Math.round(w / 2) + 'px';
imageStyle.marginRight = -Math.round(h / 2) + 'px';
imageStyle.left = imageStyle.top = '50%';
imageStyle.width = Math.round(w) + 'px';
imageStyle.height = Math.round(h) + 'px';