average of PIL images

[vaneric]

hi
i have a set of RGB images of diff faces (of people )as a 2 dim
numpyarray
..something like
threefaces=array([[xa1,xa2,xa3],
       [xb1,xb2,xb3],
       [xc1,xc2,xc3]])
where xa1,xa2,xa3 are  tuples each representing rgb values of a pixel
of first image ..

i need to create the average face image and display it.problem is i
need to calculate (xa1+xb1+xc1)/3  etc to calculate avearge value of
each pixel.how can i do this calculation.do i need to convert the
r,g,b in to a single value for each pixel? the average value of a
pixel will be a float isn't it? how can i create a PIL image from
this?
any help,directive greatly appreciated
eric