image source fix (#549)

* create asset folder

* add developers image

* add new image with proper hosting

* remove placeholder file that created asset folder

Author: kylemccollom

guides/get-started/assets/developers.png

@@ -1,7 +1,7 @@
 ---
 title: Stellar Network Overview
 ---
-![Stellar Ecosystem](https://www.stellar.org/old-content/2016/06/Stellar-Ecosystem-v031.png)
+![Stellar Ecosystem](assets/developers.png)
 
 Using the Stellar network, you can build mobile wallets, banking tools, smart devices that pay for themselves, and just about anything else you can dream up involving payments! Even though Stellar is a complex distributed system, working with it doesn’t need to be complicated.