1. A non-transitory storage medium storing an image processing program causing a computer to execute processing of: obtaining an image, photographed with a camera, of at least four markers disposed in a real space; creating vectors, which correspond to the at least four markers, from the camera according to internal parameters including information about a focal length and an optical axis position of the camera and to a positions of the at least four markers on the image; selecting a reference marker from the at least four markers; calculating a inner product of a vector for the reference marker and a vector for each of remaining markers other than the reference marker; canceling, for markers, other than the reference marker, for which a sign of the inner product is negative, use of a negative sign included in an equation representing a solution of a quadratic equation that obtains a distance between the camera and a marker other than the reference marker by using the reference marker; creating sign patterns, for all markers other than the reference marker, by combining signs included in equations corresponding to the markers other than the reference marker according to a result of cancelling the use of the negative sign; setting a first value as a distance between the reference marker and the camera; calculating one or two candidates of a distance between the camera and each of the markers other than the reference marker, for the first value, by using the first value, the quadratic equation, and the result of cancelling the use of the negative sign; calculating error between an inter-marker distance in a real space and an inter-marker distance calculated according to each of the sign patterns and the one or two candidates, for each of the sign patterns; calculating other error for each of the sign patterns when a second value, which is different from the first value, is set; determining the distance between the camera and each of the at least four markers according to the error calculated when the first value is selected and to the other error calculated when the second value is selected; and calculating a position and a pose of the camera in a world coordinate system according to the determined distance between the camera and each of the at least four markers.