Now there is a good reason to do it in Perl! I don't think it would be much slower than a script, but that might be worth testing.

#!/bin/bash

shopt -s nullglob
for image in *.{gif,bmp,jpg,wmf,jpeg,png}
do
    printf "Converting $image\n"
    bits=(`/usr/bin/identify -format "%h %w" "$image"`)

    if [ ${bits[0]} -gt ${bits[1]} ]
    then
        sizes=(600x800 81x108)
    else
        sizes=(800x600 108x81)
    fi

    ext=${image##*.}
    /usr/bin/convert -resize ${sizes[0]}! "$image" "${image/.$ext/-l.$
+ext}"
    /usr/bin/convert -resize ${sizes[1]}! "$image" "${image/.$ext/-s.$
+ext}"
done
[download]

Update: Fixed a few subtle problems in the script that surfaced with more extensive testing (not that anybody will care, this not being Perl - but I do. :)

-- 
Human history becomes more and more a race between education and catastrophe. -- HG Wells