ZimmermansWay:
I've been using this as a workaround:
1. Create a 2048 x 768 (72 dpi) image. Fill with black.
2. Crop your cover photo to 2048 x 402.
3. Paste and center the cover photo onto the black background created in step 1.
4. Save as JPEG and upload to Flickr.
Here are some examples:
www.flickr.com/groups/782343@N22/
www.flickr.com/groups/22563825@N00/
Once you have added the image as the Group cover photo, you can delete it from your Photostream.
[edited for extra clarity]
-- Flickr
