For each algorithm, the images are sorted from best (on the left) to worst (on the right). So if you look at the middle region of the graph, you see how they be

For each algorithm, the images are sorted from best (on the left) to worst (on the right). So if you look at the middle region of the graph, you see how they behave on "median" images. Another way to view the graph is: the area under the curve corresponds to the total disk space needed to store a large corpus of images in the given format. So obviously lower is better.