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.
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
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.)
The solution turned out to be avoiding php-gd altogether as I couldn't get it installed. Instead, I installed the ImageMagick Engine WordPress plugin as well as ImageMagick on the server. Then I regenerated image sizes using the plugin to create the additional sizes for images I had already uploaded.
I posted the solution on my blog if you want more detail. https://raddevon.com/articles/wordpress-image-resizing-without-php-gd/