1. An image retrieval system for searching images similar to an exemplary image from available images, the image retrieval system comprising: an extracting module operable to extract visual features of the exemplary image and transfer the visual features of the exemplary image to one or more calculating servers, wherein the visual features of the exemplary image comprise color features and shape features, and wherein the visual features of the exemplary image are extracted by dividing the exemplary image into a plurality of image blocks, calculating an arithmetic mean of gray values of pixels in each image block as the color features of the image block, calculating a ratio of a black pixel amount to a total pixel amount in the image block as the shape features of the image block, and determining overall visual features of the exemplary image according to the color features and the shape features of the image blocks; a distributing module operable to allocate image comparison tasks of the available images to the one or more calculating servers, such that the one or more calculating servers calculate similarities of the available images according to the visual features of the exemplary image and predetermined visual features of the available images, fetch image indexes of the available images from an index server, and return the similarities and the image indexes of the available images to the image retrieval system; a gathering module operable to gather the similarities and the image indexes of all the available images from the one or more calculating servers; a sorting module operable to transfer the gathered similarities of the available images to a sorting server, such that the sorting server sorts the available images according to the gathered similarities of the available images; and an outputting module operable to receive a sorting sequence of the available images from the sorting server, and output the image indexes of the available images in the sorting sequence to a client computer.