(I have CS3, but...) Try going to File > Image Preview. You should find three PNG options:
- PNG32 (full color plus alpha transparency - problematic for IE6)
- PNG24 (full color, no transparency)
- PNG8 (256 colors including semitransparent ones)
For the advantages of PNG8, see http://www.sitepoint.com/blogs/2007/09/18/png8-the-clear-winner/
You should be able to choose these formats through Export, too. I don't go that way, so can't advise you.
Also, if you last exported an image as a PNG, when you set up a slice, that's the format that is applied. It isn't one you can select from the Properties panel, though.
Thanks. I guess I didn't poke around in the menus enough. I don't use FW to generate entire pages with CSS/HTML code and all that, but mainly do my design work and only want to export the slices to use when I write my own CSS/HTML code.
(I don't use FW for code, either. Just images. But you always want to use export, not save for images for the Web.)