Here is a short script (1.6kb) to add a timestamp on all PNG pictures in a directory. It requires Python and the Python Image Library ( PIL). In order to use it, modify some parameters in the beginning to suit your needs (images directory, font file and size, etc.) and launch ./timestampFiles.py. Here is a before/after example (size of pictures is reduced to fit in this blog):

Note 1: the font is not included in the script. Here, I used FreeSans which is a true free font (GNU GPL with font exception). It is available here ( local copy, just the FreeSans font, 753kb).
Note 2: I chose to convert PNG images to JPG ones for 2 reasons. First, my capture script gives me PNG images (see previous posts). Second, I want to be able to copy all JPG pictures in a MJPEG movie. It shouldn’t be difficult to change the different file formats to suit your needs (ask me if you can’t do that).
Note 3: I guess this would also have been possible using a bash script and imagemagick, Perl or any other programming language ;-)
Looks useful, but http://www.jepoirrier.net/blogimages/timestampFiles.py is not available :(
I get an access forbidden error.
Thanks for the notice! I put the script here: https://github.com/jepoirrier/miscScripts/blob/master/timestampFiles.py
Hope this helps! Jean-Etienne