Non-square thumbnails

A lot of the images I am working with at the moment are portrait-format headshots of people. Because of the way the thumbnails are cropped and centred, most thumbnails lose the heads of the subjects.

Is it possible to specify non-square thumbnails to SimpleViewer? Possibly by specifying thumbnail dimensions on the settings page (which would have the fringe benefit of being able to control the size of the thumbnails). Or is this a limitation of SimpleViewer itself?

Failing that, another (probably harder) solution would be to somehow specify how the crop was done when creating the thumbnails. Maybe a simple choice of top/bottom/left/right/centre. That does raise the issue of whether it is done globally or on a per-image basis though.