1. An image forming system, comprising: an image forming apparatus; and a portable terminal, wherein the image forming apparatus includes a first processor that operates, when the first processor executes program instructions, as: a preview-image-data generating circuit that performs processing to generate preview image data from image data; a preview-image-data transmitting circuit that performs processing to transmit the preview image data to the portable terminal; and a duplex-printing-flag setting circuit that performs processing to set a duplex printing flag to the image data in duplex printing as a setting, the portable terminal includes a second processor that operates, when the second processor executes program instructions, as: a preview-image-data receiving circuit that performs processing to receive the preview image data from the image forming apparatus via wireless communication; a preview-image display circuit that performs processing to display an image of the preview image data; a rotation detecting circuit that performs processing to detect rotation of a main body of the portable terminal with a predetermined rotation procedure; a duplex-printing-flag determination circuit that performs processing to determine whether the duplex printing flag is set; and a composite-image display circuit that performs processing to display a composite image comprising: image data of an icon indicating that preview image data of a back side of the displayed image is available to be displayed; and the preview image data, when the duplex printing flag is set to the image data, wherein when the rotation detecting circuit performs processing to detect 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 the back side of the displayed image.