Wander through a directory, looking for jpegs, first renaming anything with a space in the name to that name with underscores and jpeg. Find the un-thumbnailed ones, create a thumbnail suffixed with ".thumb.jpg". Uses ImageMagick.

#!/usr/bin/perl
use strict;

use Image::Magick;

my $im = Image::Magick->new;

umask 0022;

my @names = @ARGV ? @ARGV : grep { -f and -B } <*>;

for (@names) {
  if (/ /) {
    my $old = $_;
    tr, ,_,s;
    $_ .= ".jpg" unless /\.jpg$/;
    ! -e and rename $old, $_ or next;
    warn "renaming $old to $_\n";
  }
  next if /\.thumb\.jpg$/;
  my $thumb = "$_.thumb.jpg";
  next if -e $thumb;
  undef @$im;
  my $ret;
  $ret = $im->Read($_)
    and warn($ret), next;
  $ret = $im->Scale(geometry => '100x100')
    and warn($ret), next;
  $ret = $im->Write($thumb)
    and warn($ret), next;
  warn "thumbnail made for $_\n";
}
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