I was trying to upload some images to a WordPress (LEMP: Linux Debian, nginx, MySQL, PHP 7.0). The images were stored correctly, but it weren’t resized to the thumbnail, medium and large sizes.
The problem: the server hadn’t installed the GD extension.
The solution was to install the extension and restart the server:
# sudo apt-get install php7.0-gd # sudo systemctl restart nginx.service
If you are using Apache
# sudo systemctl restart apache2.service