class TestReport(Report):
    _name = 'testreport.testreport'

    def __init__(self):
        super(TestReport, self).__init__()

    def parse(self, report, objects, datas, localcontext=None):
        filename = 'path_to_file/gsn.png'
    if os.path.isfile(filename):
             f = open(filename)
        datas['graph'] = [f, 'image/png']
        return super(TestReport, self).parse(report, objects, datas,
                localcontext=localcontext)

[...]