I haven't discovered any way to modify the chapter thumbs created by the MB3 server. I do notice though that sometimes the chapter thumbs are not very nice. Sometimes it catches a screen transition and the thumb is just all black.
I'd like suggest some options:
- Provide some image scanning capability built in to the server that is able to inspect the generated thumbnail and recreate it with a different time offset if it is identified as all black, all white, and so on.
- Allow an interface for the user to scan through the file and generate/select the thumb manually.