Hi,
Native API's to capture a video frame are flaky at best although they have improved in recent years. However, we don't currently have an API to do that due to that historic problem.
Ideally if your URL is online you can generate a thumbnail in the server like youtube does and then just use that as a URL. You can also use a native interface to extract a frame or file an RFE for us to add support for that (although that will probably take a while as our workload is huge).