I have the same problem as sjogro.
I have also my own webserver and no idea what GD is.
This was the solution for me:
a) First delete the picture.
b) executed this from my webserver (logged in as root):
# apt-get install php5-gd
# /etc/init.d/apache2 restart
c) Then I uploaded the picture again, and everything works fine now.
[source: https://www.cyberciti.biz/faq/ubuntu-linux-install-or-add-php-gd-support-to-apache/ ]
