#[member='JuGGa']
http://en.wikipedia.org/wiki/Transparency_(graphic)
To put it simply (and taking the liberty of over simplifying a few things), regular images have a background. In the case of these smileys, it is white. However, that creates issues like the one you now notice on the After Dark theme. Therefore, to ensure only the 'core' part of the image is all that is visible, the rest of it (the background) needs to be deleted. Such non-existent backgrounds are said to be transparent.