If I was selling apples then thieves would reduce my stock and hurt my sales. But someone who steals a photograph leaves me with the photograph. I don't think a photo thief is going to pay me for it if I make it harder to steal. I don't think a photo purchaser is going to steal it instead if I make it easier to steal. Of course there will be a few instances where these generalisations fail. I think the failures are probably rare enough that it's not worth my while going to any extra bother to try to stop photo theft happening. In fact a lawyer friend of mine said I was wasting my time trying to sell photos. Instead I should simply aim to make money by identifying photo thieves and threatening them with legal action.
I never, however, publish on Flickr or anywhere else photographs I've been contracted to take, unless to do so was a specific part of the contract, e.g. for publicity.