@wiredfool The PNG decoder only stores `transparency` as an `int` if it is a simple palette with a single transparent color (e.g. 255, 255, ..., 255, 0) see  https://github.com/python-imaging/Pillow/blob/master/PIL/PngImagePlugin.py#L73 and
https://github.com/python-imaging/Pillow/blob/master/PIL/PngImagePlugin.py#L256