`imgdiff`

The main part of the program first calculates the names of the output files:

        my @pnm = map to_pnm($_), @ARGV;

Then it converts the JPEG files to P?M files:

        for (0..1) {
          run("djpeg $ARGV[$_] > $pnm[$_]");
          my $info = run("pnmfile $pnm[$_]");
          $size[$_] = [$info =~ /(\d+) by (\d+)/];
        #  print "Size of $pnm[$_]: @{$size[$_]}\n";
        }

djpeg is provided by the JPEG reference implementation

It decompresses JPEG files, producing PPM output