CSS Style:
#watermark_box {
position:relative;
display:block;
}
img.watermark {
position: absolute;
top: 1em;
left: 1em;
}
HTML Code:
<div id="watermark_box">
<img src="854349_81704326.jpg" alt="Watermark example" />
<img src="watermark.png" class="watermark" alt="Watermark image" />
</div>