Update Screenshot (#633)

* Update Screenshot

Fixes gitpod-io/website#74

* add alt description

Author: chrifro

src/docs/browser-extension.md

@@ -5,7 +5,7 @@ Creating a workspace is as easy as prefixing any GitHub URL with `gitpod.io/#`.
 For convenience, we built the Gitpod browser extension. It adds a button to GitHub that does the
 prefixing for you. Nothing more.
 
-![Browser Extension](images/browser-ext.png)
+![Browser Extension](images/browser-extension-lense.png)
 
 We provide the extension for
   - [Chrome](https://chrome.google.com/webstore/detail/gitpod-online-ide/dodmmooeoklaejobgleioelladacbeki)

src/docs/images/browser-extension-lense.png

@@ -10,7 +10,7 @@ comprehensive, extensible, and easy to use. With deep GitHub integration, tools
 sharing, and a focus on usability, Gitpod contains your entire dev workflow in a
 browser tab.
 
-![](./images/gitpod-prefix.png)
+![gitpod-prefix](./images/prefix-lense.png)
 
 Unlike traditional cloud and desktop IDEs, Gitpod understands the context and adjusts the
 IDE automatically. For example, if you create a Gitpod workspace from a Pull Request,