1. An image processing apparatus, comprising: an input device configured to obtain a document image and to apply input processing to the document image; a common image processor configured to apply common image processing to the document image processed by the input device; a specific image processor configured to apply specific image processing to the document image processed by the common image processor; an output device configured to apply output processing to the document image processed by the specific image processor; a detector configured to detect whether the document image is copy guarded at a detection time point to generate a detection result; and a processor configured to select an application of the document image from a plurality of applications according to a user instruction as a selected application, to determine the detection time point based on the selected application, and to cause the detector to detect whether the document image is copy guarded at the determined detection time point.