window.onresize = function(){
    var img = document.getElementById('fullsize');
    img.style.width = "100%";
};

<div id="gallery" style="width: 100%">
                <img src="images/fullsize.jpg" alt="" id="fullsize" />
                <a href="#" id="prev">prev</a>
                <a href="#" id="next">next</a>
            </div>

img {
    max-width: 100%;
    height: auto;
    width: auto9; /* ie8 */
}

<div style="max-width:500px;">
    <img src="..." />
</div>