1. Using CSS to position a transparent gif overtop of the image.
<img src="myimage.gif" width="100" height="50">
<img src="transparent.gif" width="100" height="50" style="position:relative; left:-102px; top:0px;">
2. Using span tag to position a transparent gif overtop of the image.
<span style="background-image: url(images/my_image.jpg)"><img src="images/transparent.gif" width="200" height="150" border="0" alt="My protected Image."></span>
3. Use the image as a background of a table and fill the cell with a transparent gif.
<table width="100" height="50">
<tr>
<td background="myimage.gif">
<img src="transparent.gif" width="100" height="50">
</td>
</tr>
</table>