An image processing system includes: a first image processing device for performing a first image enhancement process on a source image to generate a first enhanced image; one or more second images processing device, each of which is used to perform a second image enhancement processing on a size-reduced image generated based on the source image, and accordingly to output one or more second enhanced images whose size identical to the source image; and an output controller for analyzing regional frequency characteristics of the source image to generate an analysis result, determining one or more region weights according to the analysis result, and synthesize the first enhanced image with the one or more second enhanced images according to the one or more region weights, thereby to generate an output image.