Bitmaps fading into one another

I am curious how I could make two bitmaps seem to fade into eachother. To see what I’m talking about, check out the two images (flag/gavel & people walking) at this website:

I’m doing this in Fireworks MX 2004. I’m sure this is probably an easy task, but I’m fairly new to the field, and trying to figure out some of these little tricks. Thanks.