[MightyMidget]

Ramster's galleries are nice but seriously they look like they've simply been scaled to a slightly larger size, then the center part was cropped out. Just to be the nerd of the week I'll do the same "automatically":

Aspect ratio*(150% of desired size) = 640/480*150 = 200 (thus x will be 200 pixels, y 150). Do:

convert image.jpg -resize 200 -gravity Center -crop 100x100+0+0 image_thumb.jpg

Now run this through a for loop(if you're running windows):

for %a in (*.jpg) do convert %a -resize 200 -gravity Center -crop 100x100+0+0 thumbfolder\%a

Correctly resized, and nicely centered crops (not identical to the big picture). All done.