1. An image forming system, comprising: an image forming apparatus; and a portable terminal, wherein the image forming apparatus includes: a preview-image-data generating circuit that generates preview image data from image data; and a preview-image-data transmitting circuit that transmits the preview image data to the portable terminal, the portable terminal includes: a preview-image-data receiving circuit that receives the preview image data from the image forming apparatus via wireless communication; a preview-image display circuit that displays an image of the preview image data; and a rotation detecting circuit that detects rotation of a main body of the portable terminal with a predetermined rotation procedure, and when the rotation detecting circuit detects rotation of the main body of the portable terminal while the image of the preview image data is displayed, the portable terminal displays an image of the preview image data corresponding to a back side of the displayed image.