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.
I refuse to engage in a battle of wits with an unarmed person.
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