Get an overlay image for two expressions
        Args:
            body, ExpressionOperation: The JSON encoded comparison array to be evaluated (required)
            plotShape, str: The image shape (optional)
            imageScalar, int: The scale of the image (optional)
            imageEncoding, str: The encoding of the returned image (optional)
        Returns:
            str with the raw byte data of the image
        Raises:
            CorticalioException: if the request was not successful

def compareImage(self, body, plotShape="circle", imageScalar=2, imageEncoding="base64/png"):
        """Get an overlay image for two expressions
        Args:
            body, ExpressionOperation: The JSON encoded comparison array to be evaluated (required)
            plotShape, str: The image shape (optional)
            imageScalar, int: The scale of the image (optional)
            imageEncoding, str: The encoding of the returned image (optional)
        Returns:
            str with the raw byte data of the image
        Raises:
            CorticalioException: if the request was not successful
        """
        return self._image.getOverlayImage(self._retina, body, plotShape, imageScalar, imageEncoding)