It means images that many viewers might find distasteful. I've usually seen it used in the context of graphic violence, such as in a gory slasher film. I suppose something could be considered graphic in another context other than violent, sexally graphic for instance.
Graphic refers to illustrated or drawn, as in a picture. In this context it means little imagination is needed to "fill in the blanks" be it violent or sexual images. It can also include language in its common usage for the sake of simplicity.