I'm looking for a way to list all fonts installed on a linux/Debian system, and then generate images of some strings using these fonts. I'm looking for your advice as I kind of see how to do each part, but not to do both:

To list all fonts on a UNIX system, xlsfonts can do the trick:
import os
list_of_fonts=os.popen("xslfonts").readlines()
To render a string into an image using a font, I could use PIL (Python Imaging Library) and the ImageFont class.

However, ImagesFont.load expects a file name, whereas xlsfonts gives a kind of normalized font name, and the correspondence between the two doesn't seems obvious (I tried to search my system for files named as the output of xlsfonts, without results).
Does anyone has an idea on how I can do that? Thanks!
You can do this using pango, through the pygtk package.  Pango can list fonts and render them.
Generate image for each font on a linux system using Python