Just put the names of the files in a list, and let the worker processes handle them. The example below uses ImageMagick in a subprocess to do some image conversion from an obscure format. But the same principle can be used with PIL. Just replace the contents of the processfile() function. This is a program that I use frequently to convert DICOM files (a format used in medical imaging, from an X-ray machine in this case) to PNG format.

"""Convert DICOM files to PNG format, remove blank areas. The blank erea
   removal is based on the image size of a Philips flat detector. The image
   goes from 2048x2048 pixels to 1574x2048 pixels."""

import os
import sys
import subprocess
from multiprocessing import Pool, Lock

globallock = Lock()

def checkfor(args):
    """Make sure that a program necessary for using this script is
    available.

    Arguments:
    args -- string or list of strings containing a command to test
    """
    if isinstance(args, str):
        args = args.split()
    try:
        f = open('/dev/null')
        subprocess.call(args, stderr=subprocess.STDOUT, stdout=f)
        f.close()
    except:
        print "Required program '{}' not found! exiting.".format(args[0])
        sys.exit(1)

def processfile(fname):
    """Use the convert(1) program from the ImageMagick suite to convert the
       image and crop it.

    Arguments:
    fname -- string containing the name of the file to process
    """
    size = '1574x2048'
    args = ['convert', fname, '-units', 'PixelsPerInch', '-density', '300',
            '-crop', size+'+232+0', '-page', size+'+0+0', fname+'.png']
    rv = subprocess.call(args)
    globallock.acquire()
    if rv != 0:
        print "Error '{}' when processing file '{}'.".format(rv, fname)
    else:
        print "File '{}' processed.".format(fname)
    globallock.release()

def main(argv):
    """Main program.

    Arguments:
    argv -- command line arguments
    """
    if len(argv) == 1:
        # If no filenames are given, print a usage message.
        path, binary = os.path.split(argv[0])
        print "Usage: {} [file ...]".format(binary)
        sys.exit(0)
    # Verify that the convert program that we need is available.
    checkfor('convert')
    # Apply the processfile() function to all files in parallel.
    p = Pool()
    p.map(processfile, argv[1:])
    p.close()

if __name__ == '__main__':
    main(sys.argv)

Take a look at Doug Hellmans tutorial on multiprocessing. As Björn points out, there are various issues regarding parallel processing which you'll need to get a hang of, but it can really be worth the effort.

Tip: You can use multiprocessing.cpu_count() to check the number of cores available to you.