XF 1.2 Fade Logo image to another on hover

[Verringer]

I've seen it a few times where when you hover over the logo it'll fade into another image and when you hover off of it it'll slowly fade back to it.
What's the markup that I'd put into logo_block for this?

 

[Paul B]

http://stackoverflow.com/questions/6948758/css-fade-between-background-images-on-hover

You can pick out the relevant bits from that and apply it to EXTRA.css.