From Steeple

We add

s:kind="keyframe|slide|thumb|cover" 
s:attribution=""

to <media:thumbnail>.

[edit] 1 s:kind

s:kind is used to be able to distinguish what the thumbnail is, i.e. a keyframe (the media rss default), a slide (that perhaps should be displayed synchronised with the audio track), a thumbnail (?), or a cover image (such as an album cover).

<media:thumbnail url="http://www.foo.com/keyframe.jpg" width="75" height="50" time="" s:type="keyframe|slide|thumb|cover" s:attribution="" />

[edit] 2 s:attribution

If the image is a CC image, provision for attribution needs to be made. This could be made via the s:attribution attribute.

[edit] 3 Discussion

Would it be better to have an <media:image> tag?