How do I save a custom Thumbnail for my video output file?

Hi Guys

Does someone know how I can specify / customise a thumbnail for my videos?

I Googled a few sites but could still not find a working answer.

The most common answer was that you must place your timeline marker on the frame you want Adobe to use as the thumbnail but it, as it seems to be with other people with the same question does not work.

Your help will be greatly appreciated.

Thanks, Jax.