Before you _load_ an image, which means: as you are creating it in your painting software, you should add a socalled Alphachannel to your picture in order to "paint with transparency". So you specify the (half-)transparent parts of your picture already in your image programme.
I would recommend GIMP or Photoshop. Save it as a png file and be happy with it.
I would recommend GIMP or Photoshop. Save it as a png file and be happy with it.