1. An image processing apparatus for adjusting lightness of an input image, comprising: a first processing module for generating a lightness histogram with reference to the input image and determining a maximum lightness and an average lightness according to the lightness histogram; a second processing module, coupled to the first processing module, comprising a first memory unit for storing a first look-up table, the second processing module being used for determining a first gain and a second gain according to the maximum lightness and the first look-up table; a third processing module, coupled to the first processing module, comprising a second memory unit for storing a plurality of lightness adjusting curves, the third processing module being used for selecting one of the lightness adjusting curves according to the average lightness; and a fourth processing module, respectively coupled to the second processing module and the third processing module, comprising: a first multiplier for multiplying the lightness of the input image by the first gain to generate a first temporary lightness; a processing unit for determining a third gain according to the first temporary lightness and the selected lightness adjusting curve; a second multiplier for multiplying the first temporary lightness by the third gain to generate a second temporary lightness; and a third multiplier for multiplying the second temporary lightness by the second gain to generate an output lightness corresponding to the input image.