I needed proportional thumbs for bp gallery so I changed the image source to the medium size image and used css to scale. It works but I wonder if it would be better to use the actual thumbnail files if I could. But I just can't figure out how to get them proportional. I assume the thumbnails are created and cropped on upload.
I had to do the same for activity stream thumbnails, it was trickier because the image source is printed into the activity post content.
Any advise appreciated, thanks.