4
you would need javascript for that, inserted in the theme.
google is the key to finding a script to do that.
but why bother? right click can be circumvented if you know how, and also if someone really wants an image, they just use an image grabber or take a screenshot. really there is no protection against copying images, the watermark is good, because it's an annoyance, but to prevent it fully you just can't do it..