fix conversion from Indexed PNG-image

icnsutil/ArgbImage.py

@@ -112,22 +112,14 @@ class ArgbImage:
     def _load_png(self, fname: str) -> None:
         if not PIL_ENABLED:
             raise ImportError('Install Pillow to support PNG conversion.')
-        img = Image.open(fname, mode='r')
+        img = Image.open(fname, mode='r').convert('RGBA')
         self.size = img.size
         self.channels = 4
         self.a = []
         self.r = []
         self.g = []
         self.b = []
-        w, h = img.size
-        for y in range(h):
-            for x in range(w):
-                px = img.getpixel((x, y))
-                if type(px) == int:
-                    px = (px, px, px)  # convert mono to rgb
-                if len(px) == 3:
-                    px = px + (0xFF,)  # convert rgb to rgba
-                r, g, b, a = px
+        for r, g, b, a in img.getdata():
             self.a.append(a)
             self.r.append(r)
             self.g.append(g)