#!/usr/bin/perl
use warnings;
use strict;
use Image::Magick;

my $image = Image::Magick->new;
my $wimage = Image::Magick->new;

my $rc = $image->Read('2uni2.jpg');  # base image
$rc = $wimage->Read(shift);  # image to be used as watermark

$rc = $image->Composite(gravity => "Center", 
     compose=>'Dissolve',
     image => $wimage,
      opacity => '10%',
      tile  => 1,  
      );

binmode STDOUT;
$image->Write('png:-');

# or to file for testing
#$image->Write("$0.png");

exit;
[download]

There is a way if you are on linux. You would have to write a filesystem driver (which actually isn't that difficult, see http://fuse.sourceforge.net/) and have that filesystem transparently watermark the jpgs.

But probably you were hoping for an easier solution. Here is an untested idea: You could use the apache module mod_rewrite (if you are using apache that is) to change all jpeg links to call a script instead, i.e.:

http://yourserver/a/b/thing.jpg
# gets rewritten as
http://yourserver/watermarkscript.cgi?file=/a/b/thing.jpg
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Then just write the watermarkscript.cgi that outputs the right watermarked image to STDOUT

To answer the OP: There seem to be a number of different ways to overlay or blend images. This is probably the most appropriate solution:
http://www.imagemagick.org/Usage/compose/#blend_use

Feed the images through a script, as suggested, but you only need to return the image location, not pass through the entire image, and have that location be to the watermarked version. If none can be found (-e $basepath), have the script blend one using the non-watermarked version + your watermark image.

I'd supply specific code, but I'm not very familiar with this particular library, I generally do my image manipulation with GD in PHP (ironically, the ImageMagick site is also written in PHP...)