How can I create two thumbnail sizes?

[Birchlight]

I have three Gallery Pages at http://www.wellofstars.com/Blog/ (well, more for now, but that's because I'm experimenting). I didn't want cropped square thumbnails for the paintings, but I think the sculpture thumbnails would work better if I select and crop them, since they include quite a variety of shapes. How can I select the Sculpture Gallery thumbnails and resize them as squares, ignoring proportions, without changing the thumbnails for paintings & drawings, which are proportional? I tried installing the Regenerate Thumbnails plugin, but it doesn't seem to give me an option for resizing or cropping.

Help would be much appreciated.