1. An image conversion method for performing a predetermined image conversion on a first image to generate a second image, comprising: a first step of distinguishing a shadow portion in the first image; a second step of performing a first conversion process on the shadow portion using a brightness value of a pixel according to details of the predetermined image conversion; and a third step of performing a second conversion process on a portion other than the shadow portion using a surface normal parameter which represents a surface normal of a photograph subject according to the details of the predetermined image conversion, wherein results of the processes performed at the second and third steps are used to generate the second image.