A quick Image Magick/NetPBM question

Anyone know of a way to get either Image Magick or NetPBM to create square thumbnails built from the largest square that can fit inside the center of the original image? In other words, I basically want to crop the image from a square emanating from the center and then resize that square into an 85 pixel thumbnail. As per the earlier post, I’ve started to look at Movable Type as a possible photoblog solution, but I can’t stand how it creates thumbnails for uploaded images (i.e., it distorts them if you don’t constrain the proportions, which is something I don’t want to do). I started poking around the scale subroutines in the Image module, but I can’t seem to find the right options (and the right order) to get what I want and I’m tired of working on it (or, rather, I’m just too lazy to deal with it right now).

This is kind of a threshold issue, because if I can’t get this working then MT is definitely not an option for me and I’ll just stick with the Pixelpost setup that I’ve already put so much time and energy into. That said, I’m 99% sure this is doable, and easily.