1. An image processing apparatus comprising: an image data acquiring portion configured to acquire first image data representing a color image; an image determining portion configured to determine whether or not the image represented by the first image data is an image mainly composed of black characters; and an image data converting portion configured to convert the first image data to second image data by converting the first image data to binary image data when the image determining portion determines that the image represented by the first image data is an image mainly composed of black characters.