The get_thumbnail and get_preview endpoints return the requested thumbnail or preview data directly in the response body (as application/octet-stream). Exactly what you do with the data is up to you; for instance, you might save it to your server and serve it back to the web browser, convert it to a different format, etc.
The Dropbox API doesn't offer a way to retrieve links for thumbnails or previews instead, but I'll pass this along as a feature request. I can't promise if or when that might be implemented though.
