Additional image sizes are not being generated

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.

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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.)

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.
http://raddevon.com/wordpress-image-resizing-without-php-gd/