def to_json(value, **kwargs):
        """Convert a PNG Image to base64-encoded JSON

        to_json assumes that value has passed validation.
        """
        b64rep = base64.b64encode(value.read())
        value.seek(0)
        jsonrep = '{preamble}{b64}'.format(
            preamble=PNG_PREAMBLE,
            b64=b64rep.decode(),
        )
        return jsonrep