After that, you will need to be more clever. The practice of hiding messages in plain sight (such as in the slight increase of noise in a particular picture) is called steganography. If you google for it, you will get loads of theory, and some code, including steghide, a program designed to hide messages in pictures.

Your script would need to call steghide before each download, and hide the string "downloaded from foo by user bar".

I suggest including the name of the site just so the miscreant doesn't try to explain that he signs all "his" pictures with his username.

I'm sorry, but I couldn't find perl bindings for steghide.

You may want to hack around with Image::Magick, or (to make things a little "easier"), run your image through something like stamp.

Update: The options above will create visible text on top of your images, JPEG::JFIF, or JPEG::Comment can edit the "hidden" data. Then again, if you can change it, so can the abuser. I'd go for a small visible text line (that will scare the abuser a lil' too :)

HTH

Of course even something like that's not going to survive a crop or someone going in and removing the image tags (as you noted). To really do something like this you'd want to look into something like Photoshop's watermarking plugin that's much harder to remove. It depends on how determined you are to mark the image (and how determined the swiper is to swipe ("Swiper no swiping!"</Dora the Explorer>; yes, I have young children why do you ask? :)).

This is an implementation of what was suggested in the nodes above:

#!/usr/bin/perl
# filename: marker.pl

use JPEG::Comment;
use File::Slurp;
use CGI;

use strict;
use warnings;

my $images = '/path/to/images';
my $q = CGI->new;
my $username = $q->remote_user;
my $path_info = $q->path_info;
my $filename = "$images/$path_info";

if (-e $filename) {
  my $image = read_file($filename);
  print $q->header('image/jpeg');
  print jpegcomment($image, "Downloaded by $username");
} else {
  print $q->header( -status => '404' );
}

exit;
[download]

You didn't binmode STDOUT before writing that file to it.

Yes, he did; it just looked like this: use CGI;

OTOH, when we're talking about thiefs, I don't think this kind of mark is even remotely robust.