Yeah, if the artists had access to the images and they were free to use or they paid the licenses, there's nothing wrong with doing this. Using image references is incredibly common and not a sign of being lazy, but having a super-tight schedule during which you can't just go and paint every single person and object there. Though ideally you'd rather paint all the basic shapes by yourself, then add references to increase the detail while still keeping the original layout and balance you sketched out, of course.
Edit: Woah, this thread is kinda old. Sorry for the bump, dunno why I posted here lol (guess I found this thread searching something, left it here and then when I came back now I thought it was recent lol)