sharing image tweaks

* remove text on image
* center vertically
* change urls from /sh/ to /u/

![generate_image 3](https://cloud.githubusercontent.com/assets/7763/5273977/af9b2a28-7a43-11e4-84f1-aa0efbef2c4e.png)
![generate_image 4](https://cloud.githubusercontent.com/assets/7763/5273978/af9dae4c-7a43-11e4-8e8e-6228ad820ff1.png)
![generate_image 5](https://cloud.githubusercontent.com/assets/7763/5273979/af9fef90-7a43-11e4-9a90-f59ecfba41d3.png)