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; wherein the image determining portion determines whether or not the image represented by the first image data is an image mainly composed of black characters based on distribution of brightness or density of pixels of the image represented by the first image data; and wherein the image determining portion generates a histogram in which classes represent the brightness or density of the pixels; counts, as a first number of classes, a number of the classes where a frequency indicated in the histogram exceeds a predetermined first threshold; counts, as a second number of classes, a number of the classes where the frequency indicated in the histogram is less than a second threshold that is smaller than or equal to the first threshold; and determines whether or not the image represented by the first image data is an image mainly composed of black characters based on a ratio between the first number of classes and the second number of classes.