When I upload an image through the admin interface, the image is only saved at full-size. The other sizes are configured to their default values in Settings > Media. The automatic resized versions are not being generated. I have a feeling this is due to not having a specific PHP extension installed/enabled, but I don’t know what extension WordPress uses or how to install/enable it.
2 Answers
From https://make.wordpress.org/core/2012/12/06/wp_image_editor-is-incoming/ :
Imagick support requires Imagick 2.2.0+ compiled against Imagemagick 6.2.9+, for full support. If the required functions aren’t available, WordPress will default back to GD.
If you can, create a PHP file somewhere on your server with this content: <?php phpinfo(); ?>. Then load the page and see if there is a section for either imagick or gd.
From mine:
gd
GD Support enabled
GD Version bundled (2.0.34 compatible)
etc.
(It’s probably best if you delete or disable your <?php phpinfo(); ?> file afterwards, as it may supply plenty of valuable information to potential attackers.)