SO 100% height

HTML

<div class="mainImg">
    <!--<img src="http://upload.wikimedia.org/wikipedia/commons/5/5e/Big_Wood,_N2.JPG" alt="woods" />-->
</div>

CSS

html {
    font-size: 10px;
    line-height: 10px;
}

body {
    margin: 0;
    padding: 0;
}

.mainImg { /* Remove background and background-size to see how it is with an img tag in the html */
    width: 100%;
    /*overflow: hidden;*/
    border:3px solid #ccc;
}

JavaScript

var wH = $(window).height();
    
    $('.sideBar, .mainImg').css({height: wH});