Libjpeg is required for getting jpeg support on Mac in Python Imaging Library. To get it and install, do

Manual#

$ curl -O http://www.ijg.org/files/jpegsrc.v8c.tar.gz
$ tar zxvf jpegsrc.v8c.tar.gz
$ cd jpeg-8c/
$ ./configure
$ make
$ sudo make install

Note: You may have to create /usr/local/include

Port#

$ port install jpeg