Homepage: fade in background?

ChancyRat

Inspired by Aaron telling me I can have to overlapping background images for the site, my next question is based on this beautiful site example:

http://www.markandtracyphotos.com/

I think this is a slideshow? Which I see has the option to fade in.

However I would like one of the two background images I will have, to fade in.

Hoping I'm not asking for night and day at the same time...