You can also modify your code so that a 1px x 1px transparent gif takes on the same size as the image you are trying to protect and is layered in front of it.
Someone who really wants your image can still grab it, but if they right-clcik on the image they will be trying to download the transparent gif (i.e. transparent.gif).
Example:
<div style="position: absolute; left: 5px; top: 5px;">[img]image.jpg[/img]</div>
<div style="position: absolute; left: 5px; top: 5px;">[img]transparent.gif[/img]</div>
This layers the transparent image over the image you want to protect. When a user right-clicks on it and clicks Save Picture As, it will be saving the transparent.gif image. Again, this can be circumvented. However, most users won't take the time to figure this out and other users will not be annoyed with alerts when they try to right-click something.