Yes you can set a thumbnail for a playlist please just click grab thumbnail within s3bubble admin on your selected video, you then have a option to either let s3bubble automatically grab a screenshot half way through the video, or you have the option to upload a personal thumbnail.
If you are experiencing problems with thumbnails being grabbed please see below.
We think it may be to do with the permissions in your IAM account each policy needs to allow the service.
If you go to IAM the goto users and click on a user that has the credentials you are using. Go to the permissions tab and then click on attach user policy and find the S3 policy under select policy template.
This should fix it.
Hope that helps, please let us know how you get on.