I wrote a similar script to create thumbnails of images, but instead of using GD, I used some of Tk's functionality. Here's the relevant snippet:

use Tk;
use Tk::JPEG;
use Image::Size;
#..snip..

my $main = new MainWindow;
my $canvar = $main->Canvas()->grid;
#..snip..

# $im_format is either "gif" or "jpeg"
# $_ is full path to file
my $image = $canvar->Photo('-format' => $im_format, -file => $_);

# Using the Image::Size module
my ($x, $y) = imgsize($_);

# An easy number for me to use - YMMV       
my $scale_factor = ceil($x / 100);

# This will be the thumb
my $newimg = $canvar->Photo();

# This is where the magic happens
$newimg->copy($image,'-subsample' =>$scale_factor);

# Write it to file
$newimg->write($thumb_name, -format => $im_format);

# Housekeeping
$image->delete();
$newimg->delete();

#..snip..
[download]

Not super elegant, but it works!

Guildenstern
Negaterd character class uber alles!