What I see in the gray picture is what is called digital noise. In digital pictures, the brightness is represented by a number between 0 and 255, maybe like 203 or 204. But the thing is, there's no 203½. So, to get you to see a smooth change of brightness, there are patches of 203s and patches of 204s together. You don't see it until you zoom in. There are so many pixels, literally millions of pixels, (maybe 48 megapixels!) that every so often the tiny changes will be kind of in the form of Mickey Mouse or an airplane or something. With so many pixels, it just has to happen-- like rolling snake eyes with a pair of dice 5 times in a row. You never ever see that, because you're not rolling 50 million times! Here, every picture has 50 million shots.
Notice that the places that you've pointed out are all in areas of slight gradations of brightness-- the sky, clouds, shadows-- all just what would be expected.
Does that help?