Could you add "disjoint" to the title? It's so close to containing the gist of the problem. If anyone else goes from the title to the solution, it'll save some confusion.
The way JPEG works is by dividing up the image into 8x8 pixel blocks, then transforming each block using the discrete cosine transform (DCT). If you look closely at a JPEG file, especially at low quality settings, you can see the edges of the blocks. This means that artifacts have been introduced during the compression of the image.
PNG however, is a lossless image compression format, meaning that no artifacts will be introduced by saving and loading an image in PNG.
You have a very high quality JPEG in this case, even bigger than the PNG. So there aren't visible artifacts in this case.
It's more of a general principle.
Yeah, you are right, the example number list in the post was not that awesome. If I am free at any time, i will modify the example. Just refining the diagram will be quite painful.
