ImSim: Image Similarity

[n00m]

Let me present my newborn project (in Python) ImSim:

http://sourceforge.net/projects/imsim/

Its README.txt:
---------------------------------------------------------------------
ImSim is a python script for finding the most similar pic(s) to
a given one among a set/list/db of your pics.
The script is very short and very easy to follow and understand.
Its sample output looks like this:

  bears2.jpg
--------------------
  bears2.jpg    0.00
  bears3.jpg   55.33
  bears1.jpg   68.87
    sky1.jpg   83.84
    sky2.jpg   84.41
     ff1.jpg   91.35
   lake1.jpg   95.14
  water1.jpg   96.94
     ff2.jpg  102.36
  roses1.jpg  115.02
  roses2.jpg  130.02

Done!

The *less* numeric value -- the *more similar* this pic is to the
tested pic. If this value > 70 almost for sure these pictures are
absolutely different (from totally different domains, so to speak).

What is "similarity" and how can/could/should it be estimated this
point I'm leaving for your consideration/contemplation/arguing etc.

Several sample pics (*.jpg) are included into .zip.
And of course the stuff requires PIL (Python Imaging Library), see:
Home-page: http://www.pythonware.com/products/pil
Download-URL: http://effbot.org/zone/pil-changes-116.htm