1. An image forming apparatus, comprising: an image scanning device configured to scan a document image of a document; an image type identifying unit configured to identify a type of the document image scanned by the image scanning device; an operation detecting unit configured to detect an image quality adjustment operation by a user; a preview processing unit configured to select one of whole preview displaying and partial preview displaying on the basis of the type of the document image and a type of the image quality adjustment operation, and perform the selected whole preview displaying or partial preview displaying, the whole preview displaying indicating a whole of the document image, and the partial preview displaying indicating a part of the document image at a higher resolution than a resolution of the whole preview displaying; and an image processing unit configured to perform image quality adjustment of the document image according to the detected image quality adjustment operation; wherein the preview processing unit (a) selects one of the whole preview displaying and the partial preview displaying on the basis of the type of the document image and the type of the image quality adjustment operation and performs the selected whole preview displaying or partial preview displaying if the operation detecting unit detects an image quality adjustment operation of a user without detecting setting a document on the image scanning device again after printing the document image, and (b) performs none of the whole preview displaying and the partial preview displaying if the operation detecting unit does not detect an image quality adjustment operation of a user within a predetermined time without detecting setting a document on the image scanning device again after printing the document image.