As of recently, my work is registered with the US copyright office, so IP lawyers are more than happy to take work pro bono because (1) win is pretty much in the bag if it goes to court and (2) legal fees are paid for by the plaintiff in addition to the statutory damages, which can be very, very large.
With this in mind, I am taking several pinners to the cleaners.
I have done everything in my power to stop the infringement. Nopin tag, htacess shenanigans - they're still pinning away from Google images, or uploading my work to their computer, and then to Pinterest. From this list of about 350 new infringement (since I've instituted these protective measures) spanning about 90 pinners, I have narrowed it down to a dozen people that richly deserve a lawsuit.
So to answer the OP... the answer is yes. You most definitely can be sued, and according to my attorneys, there is virtually no chance that the infringers will win in court. Their only option is to negotiate a settlement.
Thanks to Google maps, though, I know my pinners live in biiiiiiiiiiig houses.
I can only hope that they'll complain really loud and warn the masses that pinning can be very costly entertainment, and that it'll put a damper on the whole concept.
If 300-400 content providers were to similarly sue pinners, I think there is a chance that these unethical websites are going to see an exodus of free manpower.