1. An image processing apparatus, comprising: a memory that stores background image data and character image data, the background image data showing a background image, the character image data being separated from the background image data and showing a character image that overlaps the background image; a processing target area determining unit that determines a processing target area in the background image, the processing target area being in a prescribed positional relation with the character image; a background feature parameter extracting unit that extracts from the background image data of the processing target area a background feature parameter that includes a feature frequency showing a frequency characteristic of the processing target area, the frequency being a number of cycles completed by a periodic quantity in a unit time in a curve showing the background image data in a plane defined by pixel gradation and position; a character feature parameter extracting unit that extracts from the character image data a character feature parameter that includes a feature frequency showing a frequency characteristic of the character image; a parameter determining unit that determines a parameter for use in image processing performed on the background image data of the processing target area, according to a combination of the background feature parameter and the character feature parameter; an image processor that performs image processing on the background image data of the processing target area in accordance with the parameter determined by the parameter determining unit; and a frequency determining unit that determines whether the feature frequency of the processing target area is high or low by (A) comparing the feature frequency of the processing target area and the feature frequency of the character image, (B) comparing the difference between the feature frequency of the processing target area and the feature frequency of the character image with a predetermined threshold, or (C) comparing the feature frequency of the processing target area with a predetermined threshold, wherein the image processor performs the image processing if the feature frequency of the processing target area is determined by the frequency determining unit to be high, the image processing performed by the image processor is blurring, the parameter determined by the parameter determining unit is an intensity of the blurring, the frequency determining unit further determines whether the feature frequency of the character image is high or low by (D) comparing the feature frequency of the processing target area and the feature frequency of the character image, or (E) comparing the feature frequency of the character image with a predetermined threshold, and the parameter determining unit determines the parameter so that a blurring intensity p 1 when the feature frequency of the character image is low and a blurring intensity p 2 when the feature frequency of the character image is high satisfy p 1 <p 2 .