Would giving a value to Image cache refresh delete images when the image link has been removed from the post, if we set Image cache lifetime to store images permanently? If not, how do we find questionable images that stay on our server but have been removed?
If refeshing the image fails for whatever reason (couldn't connect, not found, etc), the image isn't updated. The system will then rely on the the lifetime (TTL) to handle when it would be removed. If that's set to permanent, then the image wouldn't be removed.
In the database, there is a failed_date and fail_count value stored for image proxy items. This also doesn't track the why -- it could be a temporary network issue, for example. But there isn't anything in the UI that exposes this information.