For the casual user, turn off (under Options –> Image) functions such as : Disable hotlinking and Cache full image.
Set the view to "Protected View".
It still allows one to see – and therefore download – the format you have in the Image View page and in the slideshow, which means people have a good photo for their website, but not for printed documents. This is where watermarking helps.
If you choose one of the themes that displays photos using Flash, photos are even harder to download, but as far as I am concerned, user experience is also much less interesting (more painful navigation, non-standard controls, and I believe not compatible with ipad/iphone).
By the way, even my 80-year-old mother knows that alt-PrintScreen saves a screenshot to the clipboard, ready to be pasted in Word.