saving file permission denied on windows

[zombek at gmail.com]

I'm making a small program which takes a folder with images and
generates optimized normal-sized images and thumbnails using Python
Imaging Lbrary (PIL). The problem is here:
....
os.mkdir(self.output)
....
img = Image.open(os.path.join(self.dir,file))
img = img.resize(self.imgSize)
# and here comes the error
img.save(self.output, "JPEG", optimize=1)

IOError: [Errno 13] Permission Denied
"D:\\Szymek\\python\\pythumb\\images\\proba"

I don't know what's going on, I didn't have any problems with it in the
past. I tried to save it to a pre-made directory but the effect is the
same, so I don't think it's connected with os.mkdir.