1. A non-transitory computer readable medium storing an image processing program that is executed by a computer which displays a synthesized image of a real world image and a virtual image on a display device, the image processing program causing the computer to perform functionality comprising: acquiring a real world image captured by a real camera; defining first and second virtual regions in a virtual image based on a boundary surface adjoining the first and second virtual regions, the first virtual region located inside the boundary surface and the second virtual region located outside the boundary surface; generating a synthesized image by synthesizing the real world image and the virtual image depicting a first virtual object, in such a manner that the first virtual object appears to be present behind the real world image when the first virtual object is in the second virtual region; and displaying the synthesized image on the display device, wherein a portion of the first virtual object, appearing to be present behind the real world image, is depicted as a silhouette of the first virtual object.