I've seen allot of people asking for thumbnail generators and
scripts solving some problem similar to this.
I also needed to do this recently so I spent some minutes writing a script
for it.
It's not the most interesting/efficient thing but it cuts some time and
probably it will help somebody.
I tested it using Windows.It is dependant on Image Magick.
The exact behaviour is the following:
1)get all files in directory
2)traverse each and every one of them and see if it has usual image extension
3)if 2 happens then it gets dimensions of image
4)if height>width then it uses some predefined resolutions to which it creates new
copies of the image scaled down to the predefined resolutions.
5)if width>height it reverses the predefined resolutions height with width and does the same
as described in step 4

use strict;
use warnings;
my @Ll=(800,600);
my @Ls=(108,81);


sub get_dimensions {
    my ($N)=@_;
    my $o = `identify $N`;
    my ($X,$Y) = $o=~ /\ (\d+)x(\d+)\ /;
    
    return ($X,$Y);
}
sub convert {
    my ($X,$Y,$N,$M)=@_;
    #X,Y dimensions to convert to
    #N-original name of file
    #M-name of converted/modified file

    
    my $cmd = "convert -resize %s %s %s";
    $cmd=sprintf $cmd,"$X".'x'."$Y!",$N,$M;
    `$cmd`;
    #print $cmd;
}

sub convert_small_large {
    my ($N)=@_;
    my ($name,$ext)=split '\.',$N;
    my ($Xi,$Yi)=get_dimensions $N;
    if($Xi>$Yi) {
        convert @Ll,$N,"$name-l.$ext";
        convert @Ls,$N,"$name-s.$ext";
    } else {
        convert reverse @Ll,$N,"$name-l.$ext";
        convert reverse @Ls,$N,"$name-s.$ext";
    };
}



my @files = <*>;
print join "\n",@files;

for (@files) {
    print "converting $_ ...\n";
    my $namelow=lc $_;
    next unless $namelow =~ /(gif|bmp|jpg|wmf|jpeg|png)$/;
    eval {
        convert_small_large $_;
    };
}
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