They can take a fairly long time to generate, and they also make backups.json a bit hard to read. Having them on by default makes sense, but there doesn't seem to be a way to turn them off entirely (even for users with manually managed servers that will never actually see the preview images rendered).

It would be nice to be able to say something like "preview_dimension": "none" (to complement the existing "all" value) to disable previews entirely.