Lots of webtoonists are using the PNG format these days. Here’s a good tutorial page on how to use PNG for your webcomic.
Two key advantages of PNG over GIFs is that PNG is open source so there will never be a patent holder to chase after you and if used correctly, PNG has better compression than the GIF format.
Couple good comments on formats here.
I post whatever looks best and loads quickly. For my B&W work, for some reason, a GIF beats a PNG all hollow, while the opposite is true for the color work. However, I think it’s a size issue more than anything– small files compress better under GIFs and most of my comics are small.
If they ever try to go after me for using GIFs (bad idea-I’m broke) then I’ll just re-optimize the suckers and post non-proprietary format.