All a screen monitor will display at is 72 Dpi.You can set the Dpi in image size. If you take a image and change the physical dimensions AND keep the same resolution, Photoshop interpolates and creates/subtracts new pixels/dots in order to fill in the spaces. If you keep the same physical dimensions and change the resolution, PS will either create or subtract dots depending on which way you go, up or down.It gets pretty technical sometimes and I would suggest just creating a image at a certain physical dimension and resolution and leaving everything else up to someone in production. That way you do not have to back peddle and try to straighten everything out like in this situation.