If what you say is correct then why aren't we hearing about hollywood stars suing porn sites? I get email all the time saying they have the pics of Britney, Jennifer Lopez, Julia Roberts, etc.
You know darn well that they don't have "real pictures", but doctored ones. Although the tabloids do get sued from time to time by the stars for altered photos, they still do it.
That's simply a matter of what's practical from a business and legal standpoint. Those images are usually done anonymously. It isn't practical for a Britney Spears or a Sandra Bullock to spend money tracking down some geek with a copy of Photoshop in his parents' basement. As you said, they sometimes catch tabloids doing it and thus it's easy to pin the blame and the legal fees on someone.