Python dynamic inheritance: How to choose base class upon instance creation?

I would favor composition over inheritance here. I think your current inheritance hierarchy seems wrong. Some things, like opening the file with or gzip have little to do with the actual image format and can be easily handled in one place while you want to separate the details of working with a specific format own classes. I think using composition you can delegate implementation specific details and have a simple common Image class without requiring metaclasses or multiple inheritance.

import gzip
import struct


class ImageFormat(object):
    def __init__(self, fileobj):
        self._fileobj = fileobj

    @property
    def name(self):
        raise NotImplementedError

    @property
    def magic_bytes(self):
        raise NotImplementedError

    @property
    def magic_bytes_format(self):
        raise NotImplementedError

    def check_format(self):
        peek = self._fileobj.read(len(self.magic_bytes_format))
        self._fileobj.seek(0)
        bytes = struct.unpack_from(self.magic_bytes_format, peek)
        if (bytes == self.magic_bytes):
            return True
        return False

    def get_pixel(self, n):
        # ...
        pass


class JpegFormat(ImageFormat):
    name = "JPEG"
    magic_bytes = (255, 216, 255, 224, 0, 16, 'J', 'F', 'I', 'F')
    magic_bytes_format = "BBBBBBcccc"


class PngFormat(ImageFormat):
    name = "PNG"
    magic_bytes = (137, 80, 78, 71, 13, 10, 26, 10)
    magic_bytes_format = "BBBBBBBB"


class Image(object):
    supported_formats = (JpegFormat, PngFormat)

    def __init__(self, path):
        self.path = path
        self._file = self._open()
        self._format = self._identify_format()

    @property
    def format(self):
        return self._format.name

    def get_pixel(self, n):
        return self._format.get_pixel(n)

    def _open(self):
        opener = open
        if self.path.endswith(".gz"):
            opener = gzip.open
        return opener(self.path, "rb")

    def _identify_format(self):
        for format in self.supported_formats:
            f = format(self._file)
            if f.check_format():
                return f
        else:
            raise ValueError("Unsupported file format!")

if __name__=="__main__":
    jpeg = Image("images/a.jpg")
    png = Image("images/b.png.gz")

I only tested this on a few local png and jpeg files but hopefully it illustrates another way of thinking about this problem.