Fix transparency issue in png and gifs

http://www.akemapa.com/2008/07/10/php-gd-resize-transparent-image-png-gif/

https://groups.google.com/forum/?fromgroups#!topic/croogo/F00CZBvmf7g

http://croogo.lighthouseapp.com/projects/32818/tickets/396-resized-png-attachments-are-plain-black