It doesn't have to be Photoshop. It could be Gimp, Krita or some other graphics program where you can actually draw.
But let's clear up the misunderstanding. What he does is not photobashing. He just traces faces. Maybe something else, but not that much in relation to the whole work. First of all, it will be hard for him to find a photo for all his backgrounds, monsters, their stomachs and tentacles, some of the poses of the characters. I'm sure he draws most of it from his head.
This comment about applying filters is just slander based on ignorance. Even the faces he draws are not recreated 1 to 1, which is what filters would do. Not to mention the tragic quality of these photos, overexposure and lost detail. I myself also when I was editing his works I also looked at his work carefully enough to see the methods in which he draws certain things. It was a good lesson for me. I would have seen it if it was a filter.
Ninja draws and his oil paintings, and track record are enough to prove that he has the skills to do it. If you want to draw at his level, you have to devote many years of practice in drawing.