def getRastersAsPngs(self, session, tableName, rasterIds, postGisRampString, rasterField='raster', rasterIdField='id',  cellSize=None, resampleMethod='NearestNeighbour'):
        """
        Return the raster in a PNG format
        """
        # Validate
        VALID_RESAMPLE_METHODS = ('NearestNeighbour', 'Bilinear', 'Cubic', 'CubicSpline', 'Lanczos')

        # Validate
        if resampleMethod not in VALID_RESAMPLE_METHODS:
            print('RASTER CONVERSION ERROR: {0} is not a valid resampleMethod.'
                  ' Please use either {1}'.format(resampleMethod,
                                                  ', '.join(VALID_RESAMPLE_METHODS)))

        if cellSize is not None:
            if not self.isNumber(cellSize):
                raise ValueError('RASTER CONVERSION ERROR: cellSize must be a number or None.')

        # Convert raster ids into formatted string
        rasterIdsString = '({0})'.format(', '.join(rasterIds))

        if cellSize is not None:
            statement = '''
                        SELECT ST_AsPNG(ST_Transform(ST_ColorMap(ST_Rescale({0}, {5}, '{6}'), 1, '{4}'), 4326, 'Bilinear')) As png
                        FROM {1}
                        WHERE {2} IN {3};
                        '''.format(rasterField, tableName, rasterIdField, rasterIdsString, postGisRampString, cellSize,
                                   resampleMethod)
        else:
            statement = '''
                        SELECT ST_AsPNG(ST_Transform(ST_ColorMap({0}, 1, '{4}'), 4326, 'Bilinear')) As png
                        FROM {1}
                        WHERE {2} IN {3};
                        '''.format(rasterField, tableName, rasterIdField, rasterIdsString, postGisRampString)
        result = session.execute(statement)
        return result
Return the raster in a PNG format