Two words sum it up: Image Distortion.
There are three relevant types of image distortion when it comes to wallpapers.
These distortions are on the pixel level, so if you're having trouble seeing them in the following images, you have to open the image in its own tab, magnify it to full resolution, and lean in to your screen.
1. Graininess - This one is most widely known and easiest to recognize. Here are some images of yours that may be wonderful images, works of art, taken with expensive cameras, etc, but are, very unfortunately, grainy:>>7027958>>7028025 - make sure you are viewing the full-res version, then look at the sky>>7027999 - look at the "misty" mountains in the background on the shadowy side of the image - at least some of this mist is really just grain.
Graininess is only present in pictures of the real world, as opposed to digitally generated images. It is often easiest observed in areas of pictures that have clouds, sky, or low light.
Sometimes graininess is part of the artistic effect of the image, like an instagram filter, and if that's your cup of tea, that's fine. But I personally don't typically appreciate graininess, and more often than not it just means it's a low-quality image, wholly unfit to span across my desktop.