We propose a method to solve the image segmentation problem for a complicated picture by finding the optimal quadtree decomposition in order to apply to an algorithm for computing the maximum weight region to each subgrid in the decomposition. The time complexity of our algorithm is O(N3.5) time for an n x n pixel grid containing N = n2 pixel. We further give an efficient version of the algorithm in order to deal with high resolution pictures, and conduct an experiment to demonstrate visual quality of our segmentation method.