Hop onto your image and head off into the way back time. This is the point from which you need to start brainstorming! Take a picture in your mind of what an aspect of the world would be like if certain events happened. Then use this "what if" to create a story that explains why the natural phenomenon exists. The story can be as fantastic as you want.
For example, think about a pepper grinder. What if there was a chef to the gods who lived in a beautiful green countryside but became upset one day because no one ever complimented his cooking? While wandering around, he sat under one of those beautiful green trees and wished (always be careful what you wish for in a myth) that he could somehow make the gods take notice. And suddenly in front of him was a special pepper grinder that said, "Use me, and you will be noticed." And so the chef took the pepper grinder and used it that evening as he was seasoning the gods' stew. But instead of churning out pepper, it ground out sand — more sand than the chef had ever seen! The sand kept pouring out, completely covering the beautiful green countryside. And thus the desert came into existence.