What are those blocky chunks in the clear blue evening sky? It's a JPEG "artifact". Except, unlike most ancient artifacts, these aren't worth any money, not even on the black market.
These artifacts are the result of the technique JPEG uses when compressing images. The higher the JPEG "quality" when you save it, the fewer artifacts you'll see. (And the bigger the file will be—that's the tradeoff!)
Not only do artifacts show up around gradients, but also at the boundries of high contrast objects, such as black text on a white background.
The best way to combat these is to have the camera record the image in highest quality. Also save at higher qualities after you've finished messing with the image in Photoshop.