.png also supports transparency. I don't see why mIRC shouldn't (if it doesn't already) support .png (and .mng for that matter) seeing as how it is a 100% free format.
Oh and about .gif being supported in the next version, don't count on it, .gif is not a free format. It is covered by patents which makes it illegal for the users (not Khaled, the people who use mIRC) to run mIRC without paying a royalty to Unisys.
