Edit in JSFiddle

$(function () {
    $('img').hover( function() {
        $(this).stop().animate({opacity:0.7},{queue:false,duration:200});
    }, function() {
        $(this).stop().animate({opacity:1},{queue:false,duration:200});
    });
});

<span><a href="http://www.flickr.com/photos/wneuheisel/3922107796/" target="_blank"><img src="http://farm3.static.flickr.com/2657/3922107796_ccc342077e_m.jpg" alt="Key Note" /></a><br />Key Note / william.neuheisel</span>
<p>画像にマウスホバーしてみてね。</p>