If you don't mind going outside mirc to convert, there's great freeware program called IrfanView that converts most image formats to most any other format just by saving them.
if you are just wanting a smaller file size you can always do "/drawsave @window file.jpg", by using a .jpg extension mirc will automatically save the file in jpg format. i hope this helps.
I want a png because i need to use a 255,0,255 value to set the transparency (jpg don't work with it) and bitmap is tooooooooooo big (houla, mon anglais lol) for send it by sockets