Imported From: http://groups.google.com/group/in-portal-bugs/browse_thread/thread/535bc979f699228b#
Here are some details. When cropped size is larger, then given image, then
image is not enlarger (to preserve it's quality) and larger canvas image is
created to place source image on it. Source image was containing
alpha-channel information, but created canvas wasn't. When merged together
alpha-channel information was totally lost and what was transparent in
source image became black in target cropped image.