1. An image forming system comprising: an image forming device configured to form a transparent image on a sheet by using transparent toner; a fixing device configured to fix the transparent image formed on the sheet; a size obtaining device configured to obtain information corresponding to a size of the sheet on which an image is to be formed; a storing device configured to store image data including: first image data for forming the transparent image on an entire image formable area of a sheet, the entire image formable area having a size smaller than a first size by a margin, and second image data for forming the transparent image on an entire image formable area of a sheet, the entire image formable area having a size smaller than a second size by a margin, the second size being different from the first size; a selecting device configured to select the first image data when the size of the sheet obtained by said size obtaining device is the first size and to select the second image data when the size of the sheet obtained by said size obtaining device is the second size; and a controller for controlling said image forming device so that the transparent image is formed on the sheet on the basis of the image data selected by said selecting device.