I normally use LWP::Simple to grab the contents of the image, from the remote system. You can then use Image::Size to get the size of the image, then shrink it down based on some desired height/width, then use the width/height properties of the IMG tag to display it smaller. I've used something similiar to this before.

#! /usr/bin/perl

use strict;
use warnings;

use Image::Size;
use LWP::Simple;
use CGI;

my $cgi = new CGI;
my $image = $cgi->param('image');

my $contents = get($image);
my ($width, $height, $error) = imgsize(\$contents);

#   attributes.
if (($width > 300) || ($height > 300)) {
  if ($width > $height) {
    # Image is wider that it is tall
    ($width,$height) =  (300, int((300 * $height) / $width));
  }else{
     # Image is taller than it is wide
     ($width,$height) = (int((300 * $width) / $height), 300);
  }
}

#output image
print "<IMG src=\"$image\" width=\"$width\" height=\"$height\">";
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