Maybe you should ask this to your lawyer?
To be serious: the question affords a very wide answer.
First there's a FAQ:
What is or isn't permitted on imagefapEspecially the FORBIDDEN CONTENT area lists a wide variety of sites from which you souldn't take images.
Furthermore your question touches two kinds of right that could be violated. First it's personal rights of the people shown on the pics. Usually they agreed oto be photographed so this will not really be the problem. (except "voyeur"-material) If you hide the faces you will be quite safe (quite.Not absolutely) even if the persons shown only agreed with publication to the photographer.
Second it's copyright issues. And to be honest: you re from Canada, I'm from Germany, the images are hosted elsewhere and the image was taken e.g. on the Bahamas by a Inuit who sold the right of use except the copyright to the third zebra on the right if you enter Kenia straight down the floor second door left.
So can you be liable for legal action? You can. Always. For everything. Even for the fact that Angela Merkel is a piece of dumb shit that remained from GDR and imitates an idiotic politician. If sucessfully that's another question. (In that case it isn't your fault I think so the legal action will not succeed.)
In case of those photo manips the photographer owns rights on the image. Depending on the country he lives he can give away these rights completely to somebody else or not. In Germany e.g. he can't, he only can give away the rights of usage. In Canada he can give away all rights (i guess so at least because I think it's common to British and US law)
So your question must be answered new in relation to each image. I see a lot of legal disclaimers on this site. And I think all of them are useless. You can't disclaim that you don't want be held responsible for a thing you did because by disclaiming you admit being aware of the possibility of legal violations.
On the other hand think about what a trial would mean. Of course the photographer could claim a legal suit on you. Calling the persons on the image as witnesses. In public. About an image he made for porn purposes and because somebody else manipulated it and put it to another porn site somebody (witness too) found it because he/she was browsing the net for porn. Sounds like a very good idea. The courtrooms usually are sooooooooooo empty.
HTH.