Windows has a setting that is on by default that hides file extensions. It shows Earthmoon, but the file name is actually Earthmoon.jpg. When you changed the file name to Earthmoon.jpg, the actual file name became Earthmoon.jpg.jpg. Essentially, by default, modern windows systems try to avoid letting you alter file names. So the file name should be Earthmoon, and the relative url to reach it when it's in the same folder should be Earthmoon.jpg.