Sharedaddy: Style on Admin Page

At wp-admin/options-general.php?page=sharing, some icon previews are broken.
Before:
![sharedaddy-before](https://cloud.githubusercontent.com/assets/3660715/4063411/6c47ac34-2e07-11e4-9ca9-25025c3f6dce.png)
After:
![sharedaddy-after](https://cloud.githubusercontent.com/assets/3660715/4063415/70be2c16-2e07-11e4-96aa-05de72a2db79.png)