[WordPress] Image could not be processed. Please go back and try again.

참조: http://h3x.no/2011/03/06/wordpress-image-could-not-be-processed-please-go-back-and-try-again

해결책: php5-gd 패키지 설치 후  apache2  재실행
Ubuntu:
$  sudo apt-get install php5-gd
$ sudo /etc/init.d/apache2 restart