Install the extension with one of the following commands:
$ easy_install sigal
or alternatively if you have pip installed:
$ pip install sigal
PIL is almost dead, the last release was in 2009. If possible you should prefer to use Pillow, a fork of PIL which is actively developped, with packaging improvements, Python 3 compatibility, etc.
You can install Pillow with pip install Pillow, preferably in a virtualenv. To have JPG and PNG support, you must first install the developpement packages of libjpeg, freetype2 and zlib.
For Debian/Ubuntu, this is possible with:
apt-get build-dep python-dev python-imaging
Debian/Sid users can use the experimental python-imaging package that is built from Pillow source.
For Archlinux, there is a package for sigal in the AUR which already uses Pillow.