Is there a hook which fires after all thumbnails are generated?

I want to upload the image and all generated thumbnails to another server (as backup). I only found add_attachment, but this is fired right after the image was uploaded and the postmeta table was updated.

I want to wait until all thumbnail sizes are generated and then take the files and upload them to the other server. Is there any hook which matches my expectations?

Thumbnails in WordPress can be generated by using wp_generate_attachment_metadata(), this function fires a filter after generating all the thumbnails wp_generate_attachment_metadata and the filter provides $metadata and $attachment_id to the hooked functions.

You can hook your custom function to this filter.

$metadata : Attachment metadata.
What you need is $metadata['sizes']['<size-name>'], the <size-name> is the name of thumbnail size added by add_image_size() or the default ones.
e.g.

$metadata[sizes] => Array
       (
           [thumbnail] => Array
               (
                   [file] => example_image-150x150.jpg
                   [width] => 150
                   [height] => 150
                   [mime-type] => image/jpeg
               )
           [medium] => Array
               (
                   [file] => example_image-4-300x194.jpg
                   [width] => 300
                   [height] => 194
                   [mime-type] => image/jpeg
               )
           [mysize] => Array
               (
                   [file] => example_image-4-400x400.jpg
                   [width] => 400
                   [height] => 400
                   [mime-type] => image/jpeg
               )
       )

from here you can know which sizes exist for the certain attachment, and only upload those sizes/thumbnails.

To get those thumbnails use a function like wp_get_attachment_image_src($id, $size_name) to retrieve the thumbnail urls.

(Optional) : Install the Force Regenerate Thumbnails plugin to rerun the wp_generate_attachment_metadata() for previously uploaded images too.