You need to decide as soon as you've found a rip-off what you want. We recommend first going for cash. As Samuel Johnson said: "No one but a blockhead writes but for money," and the same goes for photos.
So, first, you're negotiating. Then you're taking steps to collect on a debt from a reluctant payer. Only if that fails, or if your work is ripped off in a context that damages your reputation, should you set out to get the rip-off removed from the internet.
First, write politely. Photographer David Hoffman writes:
If there is more than one image I've found it best to only mention that I've seen one, and not let on that I know how long it's been online. It is very useful to be able to judge how honest and open the site owner is at an early stage. My initial approach is very gentle and bland: I just mention that I can't find a record of a licence for the use and ask them to let me know the terms of the licence they have. If they come clean at that point I can expect an easy ride. If not then I can give them every opportunity to stitch themselves up thoroughly before I start to shake them down.
Second, If the response is not a satisfactory offer of payment, then invoice the site owner or publisher for at least double what you would have charged them to display your work on their site, had they asked nicely. As David Hoffman says, "The sentence I cannot accept that copyright infringement can ever be a cheaper route to publication than licensed use is now hard-wired into my brain."
To work out what you would have charged them, dig up documentation of what you have actually charged similar clients for similar authorised use - and, remember, double it.
Note that the advice to double what you would have charged is a suggested bargaining position. It would be rational for them to accept it and pay up, rather than going to all the trouble and expense and uncertainty of having a court decide what they actually have to pay you.
Consider asking for extra payment on top of that if there is a breach of your moral rights or if the breach is flagrant. A prime case of breach of moral rights would, as noted, be a publication that gives the appearance that you endorse a product contrary to the NUJ Code of Conduct.
Obviously, your approach will differ depending on whether the pirate is http://some-nobody.blogspot.com with a readership of one (his mum) - or the Moloch Media Corporation with profits in the billions. The former, you might offer a chance to licence your picture for some fairly small sum, or even, possibly, let them post extracts of an article of yours on condition they link to your website so clients can find you. The latter, you'll want to charge for every minute your work has been on their site.
Third, if you have no response to the invoice, send a statement and a copy of the invoice. See Getting your money for generic advice.
And if they don't pay?
If the pirate is based in the UK, you could take them to Small Claims Court. There are, however, complications - see here - and NUJ members should contact the Freelance Office before proceeding.
If the pirate is based in the US there's not much you can do about pursuing the abuse as a debt, unless you registered the work with the Register of Copyrights. (We need an advice section on the US registration system. Coming soon.) This is another case where getting the piracy taken down is an option.
If the pirate is in the EU... the new European Small Claims Procedure will make it easier to collect money from January 2009: see this Freelance article.
In the meantime, and for non-EU countries, you have to investigate the Small Claims procedure of the country in question. NUJ members could try approaching the International Federation of Journalists member union in the country to see whether they'd be interested in helping you informally, since by pirating your work the publisher is taking work away from their members.
The National Union of Journalists must not, can not and would not wish to dictate rates or terms of engagement to members or to editors. The information presented here is for guidance and as an aid to equitable negotiation only.
Suggestions apply to contracts governed by UK law only. In any event, nothing here should be construed as legal advice.