I haven't found anything in their documentation or on the web that says yes or no. Using the Python library.
+1
A:
Apparently thumbnails can't be updated, per the docs -- media:thumbnail is not listed among the tags you can set on update. On uploading the video, yes, you can have media:thumbnail tags as part of your media:group tag which gives the video's metadata.
Alex Martelli
2009-07-27 17:27:07
While technically true that it can be set, I've found in practice that setting the thumbnail tag does not effectively set the thumbnail for a video on the initial upload.
2009-07-27 18:18:09