Thanks for doing that. So much for not suing over 1 image....
This is true, this is no longer the best rule of thumb. I think it's more about the amount of the claim than the number of images or infringements. Like Matthew commented on another thread, it's all about risk analysis. They're shooting for $30,000 in this case, so the fact that it's one image doesn't change the economics outlook for Masterfile's legal henchmen.
I don't think they'd go to court for less than a certain amount to make it "worth their while". There are also non-monetary considerations, such as the possibilities of loss of face, damage to their brand, financial blowback and setting bad precedents for their "business models". They probably go in knowing fully well it's likely to end up in a settlement that pays them enough to keep their current posturing and continue with the letter program.
They are definitely sending a statement about the former "never-for-one-image" rule. Like Matthew says, they can take people to court anytime they want and they want us to know that. However, as Matthew also says, the question is whether they can make it stick.