If you can get GD installed, then something like this will get you started:

#! perl -slw
use strict;
use GD;

our $X ||= 100;
our $Y ||= 100;

for my $imgName ( @ARGV ) {
    my $img1 = GD::Image->new( $imgName ) or warn "$imgName: $!" and n
+ext;
    my $img2 = GD::Image->new( $X, $Y, 1 );
    $img2->copyResampled( $img1, 0,0, 0,0, $X,$Y, $img1->getBounds );
    ( my $thumbName = $imgName ) =~ s[(\..+$)][.thumb.png];
    open OUT, '>:raw', $thumbName or warn "$thumbName: $!" and next;
    print OUT $img2->png;
    close OUT;
}
[download]

It resizes every image supplied on the command line to the same size (100x100 by default) and outputs them as filename.ext => filename.thumb.png.