For your information; these forums are public and not the main target for contacting support. So if you had more information in the ticket you mentioned I'd recommend to contact support over email again!
The link you provided is about creating an extension. Regarding the configuration for the file extensions.json you should provide the urls towards the image and url you can host yourself.
Easiest would be to just use the current file as an example to see what others did:
e.g. the "Dynamic Routing" one created by Trevor Fayas and Sean Wright. You see thumbnail points to devnet.Kentico.com repo (after your pull request) and source url are pointing to Trevors own github repo.
The only requirement I see is that the image should be in a sub directory setup like: "/marketplace/assets/".
The raw.githubusercontent.com domain is used to serve unprocessed versions of files stored in GitHub repositories. If you browse to a file on GitHub and then click the Raw link, that's where you'll go