None of this would be necessary, of course, if all browsers (well, IE) would support alpha PNGs properly.
But until that's done, we need to use palette altering to make the GIF/PNG shadows match the background of the page. (we'll also offer the PNG alpha shadow option, with a warning that it won't work everywhere)
Normal GIF: (generally we'll be tinting greyscale pics, but this works too)
Changing color index 1: (you've already seen this)
Tinting the whole image: (this is new)
And it works on PNG files too: (also, unified palette handling in last patch)
Next up... shadows!