Improve download help screenshots for Bug 918913

 - Update screenshots to reflect the stub installer on Windows ([Bug 918913](https://bugzilla.mozilla.org/show_bug.cgi?id=918913))
 - Use the new logo in the windows screenshots ([Bug 921264](https://bugzilla.mozilla.org/show_bug.cgi?id=921264))
 - Adjust the size of the screenshots to fit on the grid
 - Sprite the download screenshots and use JPG (faster/better)
 - Use wider/shorter aspect ration to leave more room for text below
 - Update responsive styles accordingly