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It's so easy to get around this "protection" that it's probably not worth aggrivating visitors who have other reasons to use their context sensitive menu!
When you're after simple image protection, I love the idea of placing the image in a table cell background. Then fill the cell with a transparent gif as a foreground image. This gives protection against casual image theft and doesn't disrupt the other uses of right click.
Also notice, you cannot disable right clicks in Opera.
All someone has to do is surf with JS disabled (like I do with NS 4.7) and they'll never even see a popup copyright message. It's best to have it in plain html and link to it.