'getImageDescriptor(im, xy=None)
Used for the local color table properties per image.
Otherwise global color table applies to all frames irrespective of
whether additional colors comes in play that require a redefined
palette. Still a maximum of 256 color per frame, obviously.
Written by Ant1 on 2010-08-22
Modified by Alex Robinson in Janurari 2011 to implement subrectangles.'

def getImageDescriptor(self, im, xy=None):

    if (xy is None):
        xy = (0, 0)
    bb = ','
    bb += intToBin(xy[0])
    bb += intToBin(xy[1])
    bb += intToBin(im.size[0])
    bb += intToBin(im.size[1])
    bb += '\x87'
    return bb
