Questions about texturing

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I was asking myself why game developers, do not use photo as textures to
improve reality instead making them by zero with graphic tools?

Yeah, simple answer: They do use photos. Unless the style of the game is heavily stylized and thus demands hand painted textures of course. However in some cases there are simply no perfect photos available so you have to create something yourself using Photoshop Brushes and the like. Size of the textures btw has nothing to do with it

Personally, I use a combined method. Photos as a basis and then I paint over them, add details that weren't in the photo, some subtle dark and light areas to support the shape of the object, that sort of stuff.