What do you mean by this? Are you hosting all your images on a single S3 provider and accessing them via the specified CDN?
Are you using a service listed here? Configure an S3 compatible object storage provider for uploads
Otherwise your question is somewhat out of scope of this plugin. Topic List Previews just leverages the core discourse architecture to deliver thumbnails. The images used to create thumbnails must be uploaded locally or on the allocated offsite storage specified in site settings. Other sources are not supported. The process to create thumbnails is a core discourse process and it will only act on these sources.
The default fallback image can be anywhere public, of course, but that’s the only exception.
The Topics public og tag should reflect the selected thumbnail. This is not a plugin feature, this is also core discourse.