From Timber's docs: https://timber.github.io/docs/v2/guides/cookbook-images/#work-with-external-images

Timber can work with URLs to external images. If you use a filter like |resize, |tojpg, |towebp or |letterbox and Timber detects that you passed an URL that’s not on your server, it will download that image to your server first and then process it.

External images (we call them sideloaded images) are loaded into an external subfolder in your uploads folder (i.e. /wp-content/uploads/sideloaded). You can control this behavior using the timber/sideload_image/subdir filter.