Center images using CSS

Is there a reason for not centering the images using CSS? If not, here's a pull request centering the images using CSS. :) Should feel a lot smoother.

I am not entirely sure all those `@image.show()` calls are necessary, I just replaced all the `@centerImage()` calls with `@image.show()`. `@centerImage()` called `@image.show()` anyhow.