I'd like to replace an image with another on hover, but I'm having difficulty implementing it- nothing seems to happen on hover.
<div id="container">
    <div class="content">
        <div class="left">
            <a href="#"><img src="http://dummyimage.com/307x349/000/fff" alt="tyler st salon home"></a>
        </div><!-- end left -->
        <div class="center">
            <a href="#" class="top"><img src="http://dummyimage.com/307x166/000/fff" alt="about tyler st salon" class="about_hover"></a>
            <a href="#" class="bottom"><img src="http://dummyimage.com/307x166/000/fff" alt="tyler st salon service" class="services_hover"></a>
        </div><!-- end center -->
        <div class="right">
            <a href="#" class="top"><img src="http://dummyimage.com/307x166/000/fff" alt="tyler st salon products" class="products_hover"></a>
            <a href="#" class="bottom"><img src="http://dummyimage.com/307x166/000/fff" alt="contact tyler st salon" class="contact_hover"></a>
        </div><!-- end right -->
    </div><!-- end content -->    
</div><!-- end container -->
html, body, #container, .content, .left, .right, .center {
    height: 100%;
}
#container {
    min-height: 349px;
}
#container {
    margin: 0 auto;
    width: 960px;
}
#container .content {
    margin-top: 115px;
    position: relative;
}
#container .content .left,
#container .content .center,
#container .content .right { 
    position: relative;
    width: 307px;
}
#container .content .left {
    float: left;
}
#container .content .center {
    float: left;
    padding-left: 19px;
}
#container .content .right {
    float: right;
}
#container .content .top,
#container .content .bottom {
    position: absolute;
}
#container .content .top {
    top: 0;
}
#container .content .bottom {
    bottom: 0;
}
#container .content .center .about_hover {
    width: 307px;
    height: 166px;
    background: url('img/about_hover.jpg') center center no-repeat;
}
#container .content .center .services_hover {
    width: 307px;
    height: 166px;
    background: url('img/services_hover.jpg') center center no-repeat;
}
#container .content .center .products_hover {
    width: 307px;
    height: 166px;
    background: url('img/products_hover.jpg') center center no-repeat;
}
#container .content .center .contact_hover {
    width: 307px;
    height: 166px;
    background: url('img/contact_hover.jpg') center center no-repeat;
}
 
     
     
     
     
    