I found the issue... whilst I had enabled the php-gd extension in my php.ini file, I did not actually have the php-gd package installed. As an Arch Linux user, this is fixed with..
#pacman -S php-gd
#systemctl restart httpd
All Done!
I've reported two issues in this one thread, which is rather silly. I'll [Resolve] this one and open a fresh one for the other issue.