Sorry for taking so long to check this out. I have been busy with things.
You probably want to mess with the function saveImage in /includes/functions.php. In particular the switch statement where it has all those imagecopyresampled lines.
You can find more info on that function here: http://php.net/manual/en/function.imagecopyresampled.php
You might also have to modify some other functions somewhere, but I am not really sure where. Maybe I can add that as a feature request for a future version if I ever get there.