1. An image forming system including a data processing apparatus for creating document data for an image to be formed and displaying a preview image corresponding to the document data on a display section, and an image forming apparatus for forming an image based on data acquired, wherein the data processing apparatus comprises: an instruction receiving section configured to receive an image formation instruction for the document data; an image data generating section configured to generate image data for the preview image when the instruction receiving section receives the image formation instruction; an image display section configured to display the preview image based on the image data generated by the image data generating section on the display section, the image display section comprising: a setting receiving section configured to receive settings of image forming conditions for the document data and post-image-formation processing conditions; and a modifying section configured to modify the image data for the preview image, according to the image forming conditions and the post-image-formation processing conditions received by the setting receiving section; wherein when modifying the preview image by a change in the image forming conditions and the post-image-formation processing conditions, regenerating the image data for the preview image is not performed but modifying the image data for the preview image already generated is performed; and a formation data generating section configured to generate, based on the image data for the preview image, data for image formation which is to be sent to the image forming apparatus, and the image forming apparatus comprises: an acquiring section configured to acquire the data for image formation from the data processing apparatus; and an image forming section configured to form an image based on the acquired data for image formation.