效果如下

<div class="watermark">
    <img class="logo" src="http://picture.jpg" />
</div>

.watermark {
    background:transparent url(http://watermark.png) no-repeat;
}
img.logo {
    filter:alpha(opacity=75);
    opacity:.75;
}

via:http://wange.im/watermark-with-css.html