Hello, so I'm currently studying a games design degree, for concept work we are allowed to use photo bashing, which is rough photo-manipulation from what I gather. However we strictly must reference the images used, and they must be copyright free. That is as we're told to prevent plagiarism.
As I've been browsing photo-manipulations, and digital paintings which use photo images in their construction or matte paintings, here and on artist's online portfolios, I've noticed work that is obviously using photo images however with no real reference to the original source.
So my question is here is there a type of use of photo images in art that does not formally require referencing the photo images used, or is that just only required when the art is used or presented in certain places and such?
I do hope I'm making sense.