RuntimeError: _image_module

Have you satisfied the requirements for usetex, eg, what does

    >> this report
    >> > dvipng --version
    > ahem ... all better after 'apt-get install dvipng'

    > thnx

No problem. As penance, you can write the patch to or
__init__,py (on the usetex rc param check) that issues a proper
warning if dvipng is not installed. I personally think it should be
made in matplotlib.texmanager.TexManager.get_dvipng_version. Frankly,
I don't understand why the current implementation failed

    def get_dvipng_version(self):
        if self.dvipngVersion is not None: return self.dvipngVersion
        sin, sout = os.popen2('dvipng --version')
        for line in sout.readlines():
            if line.startswith('dvipng '):
                self.dvipngVersion = line.split()[-1]
                return self.dvipngVersion
        raise RuntimeError('Could not obtain dvipng version')

OK, on a second read of the texmanager code, I do understand.
Apparently the error is that this function is not called by make_png.
I think all that is needed is to add a version check to make_png
which calls get_dvipng_version which will raise if dvipng is not
defined. Send patches to me, to this thread, to the devel list, or to
the sourceforge site.