$(function() {
    var fadedOpacity = 0.7;
    var unfadedOpacity = 1.0;
    $("img.button").animate({ "opacity" : fadedOpacity });
    $("img.button").hover(function() { $(this).stop().animate({ "opacity" : unfadedOpacity })},
                         function() { $(this).stop().animate({ "opacity" : fadedOpacity })});
});