It should be something like:

rawData = open("foo.raw" 'rb').read()
imgSize = (x,y)
# Use the PIL raw decoder to read the data.
# the 'F;16' informs the raw decoder that we are reading 
# a little endian, unsigned integer 16 bit data.
img = Image.fromstring('L', imgSize, rawData, 'raw', 'F;16')
img.save("foo.png")

Using the handbook and another SO question.

The First argument is the image mode and can be any from:

• 1 (1-bit pixels, black and white, stored with one pixel per byte)
• L (8-bit pixels, black and white)
• P (8-bit pixels, mapped to any other mode using a colour palette)
• RGB (3x8-bit pixels, true colour)
• RGBA (4x8-bit pixels, true colour with transparency mask)
• CMYK (4x8-bit pixels, colour separation)
• YCbCr (3x8-bit pixels, colour video format)
• I (32-bit signed integer pixels)
• F (32-bit floating point pixels)