Update preview image in README.md

README.md

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 Celeste is a lightweight Jekyll theme that features a minimalist, content-first design. It places your content center stage and lets your readers view them in a clutter-free environment without visual distractions. It is based on [Poole](https://github.com/poole/poole), the Jekyll butler, by [@mdo](https://twitter.com/mdo).
 
-![Celeste Preview](https://cloud.githubusercontent.com/assets/4868132/8173133/c6944c12-13fb-11e5-85b8-02e87d5f483a.png)
+![Celeste Preview](https://user-images.githubusercontent.com/4868132/48317284-981f4080-e62a-11e8-94e4-f3d7db9506a7.png)
 
 You can check out Celeste in action in [the demo site](https://nicoelayda.github.io/celeste).