Scan WordPress media library for images missing image sizes

[danielbachhuber]

As a WordPress developer, it would be helpful to have a command I could run to determine which images (if any) were missing their expected image sizes.

Because WordPress doesn't create image size variations on the fly, introducing a new image size with add_image_size() later in the life of a WordPress install can mean earlier images don't have the necessary image size variation.

[eriktorsner]

Also, looking for obsolete image sizes. Changing the default image size to new dimensions can potentially leave a lot of orphaned image file variations.

[quantegy]

Would be nice to also be able to purge unused attachments and media