How do I create a media specific custompress entry for video thumbnails?

I am attempting to create a number of different custom fields of entry so that my team can include all relevant mrss data in posts along with the URL of a video file. We have included fields such as media:hash, media:content url, media:title, media:description and media:keywords. (My hope is that these custom fields will auto populate all necessary MRSS components.)

I would like to include a thumbnail custom field of entry but am not quite sure how to achieve this. I need to include everything from the URL location of the thumbnail picture, width, height and timestamp.

Thanks,

Patrick