The only reliable solution is to use a formula to determine maximum scale ratio:

var $img = $('#content img'),

    imageWidth = $img[0].width, //need the raw width due to a jquery bug that affects chrome

    imageHeight = $img[0].height, //need the raw height due to a jquery bug that affects chrome

    maxWidth = $(window).width(),

    maxHeight = $(window).height(),

    widthRatio = maxWidth / imageWidth,

    heightRatio = maxHeight / imageHeight;



var ratio = widthRatio; //default to the width ratio until proven wrong



if (widthRatio * imageHeight > maxHeight) {

    ratio = heightRatio;

}



//now resize the image relative to the ratio

$img.attr('width', imageWidth * ratio)

    .attr('height', imageHeight * ratio);



//and center the image vertically and horizontally

$img.css({

    margin: 'auto',

    position: 'absolute',

    top: 0,

    bottom: 0,

    left: 0,

    right: 0

});