1. A computer-readable storage medium storing an image processing program to be executed by a computer of an image processing device for displaying, on a display device, a virtual three-dimensional space where there are a plurality of objects each having a collision detection region defined therefor, wherein the image processing program instructs the computer to perform: varying a size of the collision detection region of a predetermined object so that the size repeatedly increases and decreases over time, while the predetermined object is moving; determining whether or not the predetermined object is in contact with another object using the collision detection regions of the objects; and displaying, on the display device, a predetermined effect being inflicted on the object determined to be in contact with the predetermined object.