opendir(DIR, $dir) || die "can't opendir $dir: $!";
my @files =grep { /\.jpg$/i || /\.gif/i || /\.png/i || /\.bmp/i  || /\.jpeg/i} readdir(DIR);
closedir DIR;

foreach my $file (@files)
{
   my $ext;
   $file =~ m/(.[^.]+)$/;
   $ext = $1;

   $cnt++;
    rename $file, "$cnt$ext" or die $!;
    print "$file is now known as $cnt$ext\n";
}