when you save any pic as png in irfanview via the save dialog, you get an extra option window in that dialog where you can click on "save transparency color". If you check that and click on save, you can select a color from the pic to be turned into transparency. try that
. of course, as I said, this is all nicelier done with gimp where you can fine-tune your transparency layer resp. alpha channel and not just pinpoint one color which is then removed from the whole picture (like black for example in most cases).