Super simple layout has been updated to let you select the thumbnail style from the customization page.
In review, there are 3 cropping modes:
cropped -- takes the region you selected, and scales it evenly to fit within the thumbnail boundaries (say, 100x100)
stretched -- takes the region you selected, and stretches it to be exactly 100x100 (or whatever the thumbnail size is). nobody would ever use this one... and maybe I'll remove it. it's kinda dumb.
zoomed -- takes the region you selected, gets the largest sub-region it can matching the aspect ratio of the thumbnail size, centered on the focus point, and scales that down to exactly 100x100.
Zoomed is the one I'm most excited about... some really neat galleries will be possible with that. But right now, that's the one that isn't done yet. I'll post again when zoomed mode is done, along with a style that uses it well.
mart has a cool style that depends on zoomed mode, also.