How about something like this:

use Fcntl ':flock';

open my $fh, '+<', 'images.dat' or die "open failed: $!";
flock $fh, LOCK_EX;
seek $fh, 0, 0;

my @images;
while (<$fh>) {
  chomp;
  push @images, [(split /:/)[-1], $_];
}

my @ordered =
  map { $_->[1] }
  sort { $a->[0] <=> $b->[0] }
  @images;

seek $fh, 0, 0;
truncate $fh, 0;
print $fh join "\n", @ordered;

close $fh;
[download]

Update: My brain isn't taking shortcuts today. I like Aristotle's approach of passing the file read directly to map(). Didn't think of it for some reason. So a slightly simplified version:

use Fcntl ':flock';

open my $fh, '+<', 'images.dat' or die "open failed: $!";
flock $fh, LOCK_EX;
seek $fh, 0, 0;

my @ordered =
  map { $_->[1] }
  sort { $a->[0] <=> $b->[0] }
  map { chomp; [(split /:/)[-1], $_] }
  <$fh>;

seek $fh, 0, 0;
truncate $fh, 0;
print $fh join "\n", @ordered;
close $fh;
[download]

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