title: Batch conversion PNG w/ alpha-channel in IrfanView
body: <p>I have to batch convert a couple of PNG logos, each with transparent background. I used to do it with IrfanView, but now I just can't get it to work properly. I tried different options, but every one breaks transparency in some or another way.
  So, if I leave all options unchecked, I basically get PNGs with background filled with color. If I check <code>Save Transparent Color</code>, IrfanView either ask me about what color to make transparent or uses <code>Main window color</code>. But why I even have to choose color? There are no 'transparent' color in my PNGs, there are just alpha-channels, which should remain alpha and not to become white or black or whatever. Sure, I can choose some harsh-pink color (which my logos do not contain) as <code>Main window color</code>, but it feels like wrong solution.</p>