1. A computer-readable medium having stored thereon an image decoding program which, when executed by a computer, performs the steps of: storing a reference image which is a previously decoded image; receiving information including motion vector information and rounding method information specifying a rounding method used in synthesizing a prediction image; and synthesizing a prediction image by performing motion compensation using the motion vector information and the reference image; wherein the step of synthesizing a prediction image includes the step of performing either a positive rounding method or a negative rounding method for interpolating intensity values of pixels in performing the motion compensation according to the rounding method information.