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.